First off, I'm a big fan of LittleBigPlanet. This game does justice to LittleBigPlanet fully. It is extremely customizable, very good content creation tools and highly polished artwork and animation. Almost anything the original developers did in this game, you can potentially do as well. The tools are included.
Where it falls short is the actual racing. The game physics is bizarre, if you get knocked from behind you just come to an abrupt stop. The AI will always screw you over with multiple hits and always finds you when you don't have an item to defend yourself. So instead of a party karting game where you want to knock about and race freely, the odd physics and poorly balanced weapons and items makes this one of the worst karting experiences on the PS3. Even when you win, you do not feel you won because you raced well. You feel you won because you were lucky.... And this is my biggest problem with this game in that it's just no fun (especially against AI).
The multiplayer can tend to be better when there are other humans playing and without any bots. Even then there are issues with weapon balances.
It is sad, I really tried to love this game but the actual karting is a huge let down. Absolutely no complaints on any other part of this game, but I think they went too much for customization and trying to sell the game on the name of LittleBigPlanet than make the actual karting fun.
If you are looking for a fun game with countless hours of replay value and customizations, creating levels and the whole LBP experience get it by all means.
If you are looking for a solid karting game with fast racing, fun drifting and proper kart physics... skip this by all means.