For fans of South Park such as myself, playing through this game is an enjoyable and nostalgic experience. For people looking for a good tower defense game, I can point you to some much stronger alternatives.
Unfortunately I don't know much about the tower defense games available on XBL Arcade. On Playstation Network, I can recommend Pixeljunk Monsters and Comet Crash as being amazing tower defense games that really have expanded the depth people expect in the genre.
Conclusion: If you are big fans of both tower defense games and South Park, this game is for you. Otherwise I would look elsewhere.