Now, on to my second, longer post:
There are many, many great games on PSN, but many of them are also available on Steam for considerably less during sales. I know not everyone (read: hardly anyone) has a gaming rig hooked up to a living room TV, but if you do, I recommend picking up games like Rochard, Renegade Ops, Castle Crashers, and PixelJunk Eden on Steam instead of PSN. ($10 mini Blutooth dongle +
MotioninJoy + PS3 controller helps.)
Among the many, many great games on PSN, there are quite a few excellent exclusives (in this context meaning "not also on PC;" some of the games I list are probably also on XBLA) as well. I'm partial to competitive puzzle games (in case it wasn't obvious by my <3 of Critter Crunch), so I would recommend Super Puzzle Fighter II Turbo HD Remix and Tetris in addition to CC.
I've picked up a few gems on sale, like Super Street Fighter II Turbo HD Remix and...Well, okay, there aren't that many good sales on PSN. (I also bought Rochard, Lara Croft and the Guardian of Light, and the PixelJunk 3-game pack on sale, but all of those games with the exception of 2 of the 3 PixelJunk games are also available on Steam.)
I would recommend against buying full games on PSN for 2 reasons: (1) You can wait 2-6 months for the game disc price to drop like a rock on Amazon and buy it then (wish list + patience = massive savings) and (2) full games take up a big chunk of hard drive space, which may or may not be an issue for me. (I'm a demo addict, so it's an issue for me. Also, I like having at least the option of selling or trading games, even though I haven't actually done it yet.)
If you have a Move (though you probably don't) and if you haven't played Okami on the Wii (and you may have), I strongly recommend Okami HD.
Flower is beautiful and very calming, but you might want to skip buying it on PSN and just pick it up as part of the Journey collection (I forget the exact name of the anthology, but it's Journey along with Flower and other thatgamecompany games) on disc.
I thought Mega Man 9 was excellent when I played it on the Wii, but I haven't played the PSN version.
I've read that Journey and Dyad are amazing, but I haven't played the demos yet.
I did play the demo for Outland and liked it, but not enough to drop $10 or $15 on it. (Steam has warped my expectations of game pricing.)
Dead Nation is pretty meh, IMO.
Super Stardust HD is solid if you like twinstick shooters.
And I'm sure I'm forgetting some other great games, but that's my list for now.