Okay, straight to the point. The Simpsons Game is not a fantastic game. It doesn't have the depth or challenge or originality required for it to be considered a great game. Take away the license and judge the game on its own merits and it is a strictly mediocre, seen-it-all-before, by-the-numbers platform game with weak puzzle elements.
However, it is still very much worth at least renting. The game looks like it was ripped straight out of the cartoon and is very funny throughout. The game actually used the real characters' voice actors in the game rather than just sound-a-likes and they even had the show's animators to draw some of the cutscenes. This game thrives on its humour and image, and they're both done beautifully and it almost makes up for the lacking gaming experience. Almost.
It is definitely worth a play if you're a Simpsons fan, which you must be or you wouldn't be looking at The Simpsons Game, right? I wouldn't say it was definitely worth a full price purchase, but if you could rent it for a week, borrow it off a friend who did buy it, or pick it up cheaply second hand, then you will have a good few hours of fun.