The game is for fans of the show. The game works fine and is fun to play, but nothing groundbreaking here. Try renting.
From a gameplay perspective, there isn't a lot new here. By far the most fun to play is Numbuh 3, with her gliding ability. While the gameplay is far from groundbreaking and is mostly platforming/fetch quest fare, the game somehow manages to make itself fun even if it isn't going to take any risks.
All in all, I think this game's reputation is dragged through the mud unnecessarily. It certainly isn't worth $40, but I was able to locate a copy for $12, and am quite satisfied. However, if the cartoon isn't your thing, you might want to pass it by, as each of the elements in the game that make me smile stem from my enjoyment of the cartoon. And even if you do like the cartoon, be wary of overpaying for this one. Pick it up cheap and you'll get exactly what you expected--a solid and simple platformer that can be beaten in under 10 hours.