Genji is probably my favorite PS2 game of the year. Totally worth $19.99 (or less).
If you're an Onimusha fan you'll love it. As for PS2 games, it's sort of the Japanese answer to God of War. Where GoW is brash and loud and gory, Genji is the same type of action with a more entrancing zen-like quality. Both are good action games, but I liked Genji more.
Jaggies? Yep. It's jarring at first but you really get adjusted to them quickly, and the overall art direction is among the best I've ever seen. Control is silky smooth, and just fun to play - makes playing again a joy instead of feeling like you've already finished everything and seen all the game has to offer. The sound is among the best I've ever heard - crank the surround system in the opening cinematic to be blown away. It's short (6-8 hours), but you will enjoy playing it again at least once. Includes some "new game plus" type options too.
Oh, and don't bother with the strategy guide. You DON'T need it, especially considering the insance price for the Doublejump guide ($19.99? Same as the game?). Game is fairly easy and honestly somewhat linear, so you shouldn't need help. And the extras/art/etc in the guide was not very impressive to me when I flipped through the book at a store.
The linearity and low difficulty are probably the biggest complaints, so if you're looking for a vast "go anywhere" type game, Genji isn't going to fit for you. Pacing is good though, it feels more like you're being directed through a well crafted movie, but with near perfect pure action gameplay.
I've been an Onimusha fan since day 1, and I seriously wonder if Onimusha: Dawn of Dreams will be able to compete. Can't wait for Genji 2 on PS3.