I'm in the same position you are, but I've been playing my PS3 almost nonstop since I got it back in June and now vastly prefer it to my xbox 360 (probably because I grew up on PS-style stuff -- not trying to make a console war statement, just sayin'). Here's my opinion on PS3 exclusives.
Disgaea 3 - Fun, but way too much like 1 & 2. It's basically just the same thing with larger levels and new mechanics. The graphics are still bleh, there's not enough new classes, and the trophies for this game are so hard to get. The DLC seems completely pointless as the game has infinite random levels and the storyline is terrible, so there's no point in getting most of it. I'd only suggest you get this if you haven't played 1 & 2 or you're a gigantic disgaea superfan.
MAG - Also fun, but it's a whole lot more like Battlefield than Call of Duty I'd argue. The shooting and everything else isn't quite as solid and well-reacting as COD's, there's also not as many guns, but a whole lot more customization options via the skill trees. The move support is cool, but not amazing. I really like the 3 factions angle. Basically, if you like tactical big-team battles, you'll like this.
Demon Souls - Loved it, but it doesn't have a huge amount of content. There's around 20 stages, each will take you about... 30-45 minutes to finish (more or less depending on how many times you die). Farming stuff for weapon upgrading is tedious and boring, so this isn't good for doing platinum. There's a whole lot of awesome concepts to this game though, lik the fact that one of the boss fights is PVP. This is a must-play for a PS3 owner, but I think it could be rented and blown through instead of bought unless you really like the idea of PVP action-RPG gameplay (in which case it's a buy).
Infamous - Worth playing, not worth buying unless you really like superhero open world games. NOT a must-play. It's far better than prototype, the graphics are good, the storyline is OK with some goofy elements. Zero replay value in my opinion in spite of the karma system.
Killzone 2 - Awesome in so many ways, but the tank-like controls are just so depressing. The graphics are better than COD, the map design is better, and environments are more detailed. If you can soldier through the controls are don't mind a less robust progression system than in COD or MAG, you'll like Killzone 2. KZ3 looks like it's gonna be amazing, so I got KZ2 just to finish it before 3.
God of War 1 & 2 HD - I actually really didn't like this. It just seemed super overrated to me. It wasn't even dated game design aspects that bothered me, just the poor storyline and such. To me the game was a huge snorefest. I don't think I'd like 3 either.
Uncharted 1 & 2 - I love these games. I usually don't like games that are relatively linear shooters without progression systems outside of their multiplayer components, but the Uncharted seasons has a cool little ingame trophy-achievement system in addition to the actual trophy support plus the storyline (especially its execution and dialogue) is exciting and well-written. The multiplayer for Uncharted 2 is extremely fun and unique. It's like Gears of War with platforming elements only better.
Modnation Racers - I found this meh, then again I don't really like racing a whole lot, and the game just seems like an attempt at making a little big planet racing game that flopped. I can't really make what I want to make, because it didn't seem like there were enough customization options.
Little Big Planet - OK platformer, but kind of meh overall compared to expectations. The creation tools weren't robust enough. LBP2 looks mindblowingly cool, I loved the demo.
White Knight Chronicles - I think this game sucks and I love JRPGs.
Resistance 2 - OK. Online is dead, singleplayer gets a little tedious. Solid none the less. I did like it. The guns are fun to use.
PSN is where the really awesome exclusives are. I won't bother listing the ones that use Playstation Move since it doesn't seem like you bought move or are interested in it, I should note that there are a lot of cool move games on PSN.
Pixeljunk Monsters - If you like tower defense games, get this. This game is the best tower defense game I've ever played. Buy the DLC too.
Pixeljunk Racer, Shooter, Eden - I found all of these really meh.
Auditorium HD - Cool light game, awesome music, basic puzzles. Very fun, you can try out a flash version demo of it online, you should do that before deciding to buy this. Move compatible!
Mushroom Wars - I actually really like this very very simple RTS Tower Defense Hybrid (very little tower defense, it's mostly RTS with set points). Bright colors, nice music, simple, challenging gameplay.
Dead Nation - Meh. Better than zombie apocalypse, it's still a top down shooter. :|
Noby Noby Boy - WTF.
Fat Princess - Lag kills the online. Very fun game otherwise.
Hustle Kings - Cool pool game that's indepth with a lot of DLC support. It does move. Awesome soundtrack.
Planet Minigolf - I haven't gotten into this too much, but it's a solid minigolf game where you can supposedly make your own courses. I like it thus far.
