I'll be doing this briefly in sections, they will get more spoilery as you go down.
In a nutshell: Spectacular, funny and touching scenes somewhat let down by a lack of structure to the writing (though I'm sort of reserving my judgement until I see it a second time, since there was admittedly quite a bit I didn't understand this time around).
On different aspects of the show
Writing: I've been somewhat critical of GACKT's writing in the past and was admittedly apprehensive when he announced he was doing it himself instead of leaving it to people who've been writing scripts for years. I had mostly got over this and had high hopes by the time the play started, but, well. It's certainly not terrible, but I did come away feeling like this was the weakest aspect of the play. I feel like he sort of just strings the emotional and cool moments together with exposition. Of course, when he's writing an album of songs, focusing on the most emotional moments of the story exclusively is what works. But I did feel like the play could have had a better structure leading up to a climax than it did. As it was I kind of spent the show going, "Where are you going with this?" "Oh, is this going somewhere?" "Hm, not quite." "Oh hey this is actually getting pretty cool-IT'S OVER!?"
Also, he ended on a cliffhanger with a "To Be Continued". Damn you.
Acting: I was pleasantly surprised to find that Yoshitsune is not Kyoshiro Mk.2. He has far more emotional range than Kyoshiro, and GACKT pulls it off quite well. Special mention goes to Saotome Taichi, who is playing a male character, but uses his elegance and dance training from his onnagata days fabulously. Benkei's actor was in charge of some improv and interacting with the audience (there was another character who did this briefly too). Everyone else also put in solid performances.
Visuals: The costumes honestly reminded me of something out of an anime or video game. YMMV as to whether that's a good thing or not. Most of them are metallic and shiny in person. The backdrop was an LED screen like in Kyoshiro, with animated backgrounds and videos. Several scenes involved characters interacting with the backdrop, including one fight with the backdrop which was a highlight of the show. Sometimes the music was also timed with effects like water drips in the background which was a nice touch. One fight scene also made use of wire work.
Characters [HERE BE MILD SPOILERS]: Yoshitsune is far more immature than expected, sort of like what Sho would be like as a samurai. He has a lot of silly scenes and some crying scenes (heartstrings, they have been tugged), as well as some badass scenes. Another thing I really appreciated was a Female Character Who Does Things! (Tomoe) She even got a decent chunk of the fight scenes, possibly more than GACKT himself. A character I expected to find annoying but didn't, was Hiyori, the female Mononoke, who seems to mainly exist to be cute, and actually is! Benkei was very similar (and perhaps even the same actor) to the rakugo performer from Nemuri. Kage (the Mononoke, Saotome Taichi) was very elegant, androgynous and otherworldly, though I did wonder why an immortal creature like him spent so much time hanging out with a, well, dork like Yoshitsune.
ULTRA QUICK AND HALF-ASSED BASED ON LIMITED UNDERSTANDING PLOT SUMMARY [THAR BE SPOILERS]
[click if you want to spoil everything (except the bits I missed)] The play centers around a group of friends who are Mononofu (part-humans part-mononoke) Minamoto no Yoshitsune (GACKT), his cousin Minamoto no Yoshinaka, his childhood friend, crush and Yoshinaka's wife Tomoe Gozen, and his four close friends Benkei, Saburo, and the brothers Tsugunobu and Tadanobu, as well as full Mononoke Kage (who follows Yoshitsune around) and Hiyori (who follows Kage). It covers events over several years of their lives from about 1182 to 1184. One of the first events that occurs is the destruction of a village, reportedly by a Mononoke. Yoshitsune, who literally had to be dragged to go investigate, is disbelieving because Mononoke do not attack without cause. While his friends try to heal the survivors, he wanders off into the forest and encounters Kage, a Mononoke who apparently owes him a favor. Kage tries to convince him to use his powers to fight, but Yoshitsune denies he has them. However, Kage is not as easily fooled as Yoshitsune's friends- having seen those powers firsthand when Yoshitsune "saved" him once upon a time. At this, the screams of the dying and the sounds of ripping flesh and bone well up in Yoshitsune's memory, and he yells at Kage to stop. Kage states that he originally intended to "eat" Yoshitsune and obtain his powers, but instead became fascinated with him and decided to watch him for a while before chowing down. Yoshitsune speaks at length about how he feels fighting is endless and pointless, and that the existence of Mononofu can only cause strife, seemingly implying that his silliness and cowardly behaviour through the rest of the play is at least somewhat Obsfuscating Stupidity. Kage, however, thinks that humans would fight even without the Mononofu's influence. After a little more needling, Kage also mentions that there is something/someone who can manipulate Mononoke. Yoshitsune demands to know why he didn't share that information immediately before running off.
I still don't quite get this scene, but Tomoe, Yoshinaka and Benkei come across a group of men(bandits?) dressed in wolf skins, who pick a fight with them. However, they are interrupted by the arrival of an old guy who summons a bunch of shambling, child-like creatures which are apparently manipulated Mononoke. He then casts some kind of spell on them to make them into adult sized monsters with long fingers/claws, which attack Tomoe. She is unable to find any way to return them to their original form, and also discovers that they simply revive themselves if defeated. She is saved by Benkei and Yoshinaka, who kills the old guy, which also results in the Mononoke's death. Yoshitsune shows up at the last minute and falls to his knees, saying, "Our powers are not meant for this"
After this fight, Tomoe asks Yoshitsune to come meet and talk with her. When he arrives, she has a wound on her face which concerns him, and apparently it's Yoshinaka's fault. Yoshinaka then shows up to try to drag her away, which infuriates the already pissed off Yoshitsune, even more when Yoshinaka tells him not to interfere and threatens to kill Tomoe if she doesn't obey him. The situation escalates until Yoshinaka draws his sword and demands that Yoshitsune draw his daggers. Yoshitsune refuses, so Yoshinaka takes a swing at his neck, stopping at the last possible moment. Tomoe jumps in to tell both guys to calm down. Yoshitsune is considerably freaked out at almost being decapitated, enough that one of his friends mock him for "pissing himself", which Yoshitsune loudly denies doing, before resorting to sobbing in the corner about how scary it was. His friends approach him, but the first one to get close blurts out, "...wait... did you ACTUALLY piss yourself?" The rest edge away with Yoshitsune still denying it.
Some time later, Yoshinaka is assigned by Yoritomo (Yoshitsune's half brother and the current leader of the Minamoto clan they both belong to) the military task of capturing a certain location. It is a heavily protected location and Tomoe is concerned for Yoshinaka's safety. Yoshinaka, however is honored to take the job. We also find out that Yoshinaka considers himself to be the incarnation of a demon. Tomoe goes to talk to Kage, asking him what side he'd take if they were to become enemies with Yoshitsune. Kage reiterates that he follows Yoshitsune with the intention of eating him someday. Tomoe then asks him "if one day she is not around" to stop "her lord". I'm not actually sure if this means Yoshinaka or Yoritomo since I think she calls them both tono ("lord") at some point @____@;;
In the next scene, all the main group of friends (as well as Kage and Hiyori) are together drinking on the riverbank, before Yoshinaka's departure. Yoshinaka says the year he's spent with them has been like a dream.
Yoshinaka's military campaign goes extremely well at first, and it seems his self appointed title of "The Demon Yoshinaka" might be fitting. Bolstered by his victories, he continues to push forward to defeat his enemies once and for all. However, at this point, Yoritomo calls a meeting with Yoshitsune, and gives him advice on "helping" Yoshinaka. I'm actually not quite sure what happens between them, but it seems that Tomoe and Yoshinaka were expecting Yoshitsune's army to arrive as backup, and when it doesn't, they are outnumbered and Tomoe is killed. In his rage and grief, Yoshinaka swears revenge.
With Tomoe dead, Kage goes to talk to Yoshinaka and convince him that Yoshitsune does not want to fight him. He then tells Yoshinaka of the promise he made to Tomoe, and after the conversation he seemingly steps into a magical realm inhabited by En, a demonic Mononoke with some mysterious connection to Yoshinaka, who is made of shadowy shapes on the backdrop. En first appears as a shadowy figure and speaks to Kage, then bursts into a flock of swirling ravens, which he sends swooping at Kage. Kage deflects them with a fan and a dance-like fighting style. En steps up the battle by appearing as giant shadowy hands shooting beams of dark energy, finally landing a blow on Kage. Kage continues to fight, wounded, but this time the swooping ravens draw more and more blood. Finally, En manifests as a giant demonic form, threatening to kill Yoshitsune, but Kage insists that Yoshitsune is HIS prey and strikes a killing blow. En is gone, but Kage also dies soon after.
Yoshitsune goes to confront Yoshinaka, and tries to persuade him that they are friends and he doesn't want to fight, but Yoshinaka accuses him of using his friends. Yoshinaka also informs him of the deaths of Kage and En. They fight, but it quickly becomes Yoshinaka simply beating Yoshitsune senseless. Finally, the demonic power in Yoshitsune breaks free, he makes an impossible leap across the stage, and his demeanor and voice change omniously. Yoshinaka continues to fight, but this time Yoshitsune has the upper hand and stabs him in the gut. Yoshitsune briefly regains control of himself and begs Yoshinaka to run, but Yoshinaka does not attempt to. The demon then possesses Yoshitsune again, stabbing Yoshinaka several more times before making a superhuman leap into the air to deliver the final blow. Lights out.
The lights come back on to reveal a Yoshitsune sobbing uncontrollably and holding a dying Yoshinaka. Yoshitsune begs him not to die, but Yoshinaka is tired of fighting, wants to see Tomoe again, and now believing that Yoshitsune is the "incarnation of a demon", not him, he wants Yoshitsune to take his place leading the war. Yoshitsune's friends arrive, and he screams at them to heal Yoshinaka, but Yoshinaka is already dead.
Then there's a cliffhanger scene with Yoritomo stating his intention to... I've forgotten. Fight more, or fight Yoshitsune or something, then the words "To Be Continued" appear on the backdrop. Curtain call! There's also a message from GACKT stating his hope for all war in the world to cease.
After everyone takes a bow, there's actually another scene! At first, it's a repeat of the drinking scene, with everyone alive and together again, but it actually goes for longer this time. Yoshitsune tells Yoshinaka they'll always be friends, but Yoshinaka retorts that they aren't friends- they're family
EDIT: Forgot an obvious bit, at the end of this scene (drinking in the moonlight), Setsugekka starts playing and the trees in the background burst into bloom with cherry blossoms. Then confetti (snow?) starts falling onto the stage as a finale.