I've played LBP so you cannot say that I'm being a fanboy. 
OK first of all I'd like to say this: AMAZING GRAPHICS. WOW. The graphics truely are amazing and so are the physics. This truely does show off the PS3s power.
However...
The gameplay is not very exciting. Its basically like a 2-d sidescroller with good graphics and physics. It really is get from point 'a' to point 'b'. Now I know nearly all games are from points a to b but this game has it in a very bait way. Also the user created content is exciting but very little of it includes gameplay, most of it is just jumping and seeing the stuff happen. Where's the gameplay in that? You also have to spend a lot of time creating levels only for them to turn out to be roughly 2-5 minutes long. I mean whats the points in spending 2 hours if the level only turned out to be 2-5 minutes long? Overall while I absolutely love the graphics there isn't much behind it. I expect a average score of 65-80 on Metacritic.
End of '08 predictions (made by theprof00): PS3: 20m, 360: 23.5m, wii: 42m
Now Playin': PS3|Resistance 2, LBP, Uncharted and Fifa 09, PSP| Ratchet and Clank: Size matters, 360| Gears of war 2 and Fallout 3.
