I'm playing Yggdra Union, now. I tried it right after I finished Riviera the first time, but I was expecting similar gameplay, it being a sequel. The gameplay is completely different, which isn't a bad thing, but I wasn't ready for the sudden change right after Riviera. So I took a break and picked it up again recently. It's actually rather fun. Intensely strategy-based, but the battles are fun enough, and the dialogue is just so colourful.
Roswell's expression...! O: <333
The art was kind of surprising at first, because the style from Riviera changed so dramatically (most likely different artists), but I'm used to it now, and I find I appreciate the details put into the character designs. The outfits are pretty awesome. Yggdra's eyes still creep me out, though. Fortunately, she doesn't have an annoying personality, so I find her tolerable, though she does seem to be fond of saying, "I'll shall strike you down!" in her dialogue. Well, I'd be pissed too if my castle got pwned.
And another great thing about this game is that I've seen abundant enough copies of it at EB. I think it's worth buying, if I ever get a GBA/DS.