Little Big Planet is one of those games where it's up to the player how awesome the game can be. There will be people who don't like to create objects, don't like designing their own levels, and hate customizing anything in general...and those people would probably hate playing a game that requires you to become very imaginative throughout.
For the rest of you guys, this game is simply incredible.
At first, I never grasp the possibilities with such an ambitious game as Little Big Planet, but all it took was one video of some art group recreating a humongous Shadow of the Colossus level to show me how much can be done with this game. Now, I find myself wasting most of my free time trying to create imaginative levels that I will potentially share to the online community.
And what an online community it is! So many creative minds making so many levels! Some are less spectacular than others, but all are pure player-made levels that show that everyone has a creative bone in their body. I've even experienced my first online co-op play during the brief time I had on the LBP Beta with a friendly person that was patient enough to help me secure some of the more elusive items in the 2-player-only parts of the levels, all while their servers kept screwing up with our connection. We must've tried the same level at least 10 times...but we were still having fun throughout.
The single-player mode is no slouch, either, with highly-imaginative levels and tons upon tons of stuff to collect to amass your own collection with.
All in all, you only really stop playing this game when you run out of good ideas, or, as I see it, when you cease to function as a human being.