I saw it last night at midnight, and I was absolutely blown away. I went in with a mindset that, obviously, it was going to have some differences. No movie adaptation is ever 100% faithful to the books, so there will always be flaws and things that everyone who read the books will wish they included. But this is Hollywood, they have to appeal to the largest crowd and think in terms of what everyone will probably enjoy the most (and thus, earn them the most money). They also have a time constraint.... how long can they hold the viewer's interest, especially a viewer who may have not read the books and may not be completely sold on the idea to begin with?
I think they stayed pretty faithful to the book, overall. I wish there had been a few more scenes between Cinna and Katniss (so we will really have our hearts torn out when the inevitable happens in the Catching Fire movie) and Haymitch and Katniss (because I their interactions in the book were always great, and it would have been nice to have more of that feeling in the movie where they may not get along but they completely understand each other's way of thinking). I also would have liked a bit more time spent on developing Katniss and Peeta's relationship. But again, time constraints and all that.
As for scenes that they added.... the District 11 uprising gave me chills. I absolutely loved it. It made me want to stand up and start rioting too.
Seneca in that room with the berries was haunting... and brought back the scene with Snow that they added, which I also liked, where Snow commented "I like you" and told him not to mess things up (don't recall the exact wording of it). But it just shows in that type of society that it doesn't matter how well liked you are; everyone can fall. A man's life is worth nothing. No-one is safe as long as they have a mind of their own. All that matters is blind, unquestioning loyalty to the capitol.
Cato's little speech at the end...made me really feel for the character in a way I never thought I would. I'm glad they gave him that bit of depth, rather than keeping him a brutal killing machine, although I would have liked to have seen it grow. One minute he was a heartless bastard, the next he's weeping and saying how he was dead anyway and never realized it. I would have liked to see when he realized that - what made him discover that. But I guess since the movie was still, for the most part, Katniss's point of view that's why we don't get to see that.
I thought the casting was absolutely perfect. I can't think of anyone who could play the roles better. My favorite, I think, was Stanley Tucci as Caesar Flickerman. He was just brilliant --- but then again, he always is. Lawrence was superb as Katniss; her expressions really told the story all on their own. Josh Hutcherson was great as Peeta, I just love his sweet little puppy-dog eyes. <3 But he still has that unfaltering inner strength and goodness, and I could see his love for Katniss.
Overall, loved it.