The PSN games I have downloaded are PAIN, Pixel Junk Monsters, Super Stardust HD and Flower
Flower is pretty good. You just float around controlling a petal threw the air tilting the six axis. Its a relaxing game and it looks great but it dose get boring pretty quick. Pixel Junk Monsters is alright and you get lots of levels to play. But PAIN gets crap pretty quick tbh. I think the best is Super Stardust HD.
There are demos of PJ Monsters and SS HD on the store so I would try them out. SS HD is the best PSN game imo, but Flower is worth getting if you are in the mood for a chill out game