If anyone even remotely likes tower defense games, GET DEFENSE GRID. It is quite possibly the most polished, balanced, innovative and beautiful games in its genre. It has a storyline that is enough to keep you interested, provided by an ingame computer narrator who is actually quite a good voice actor. The controls are simple, the game isn't terribly graphically demanding (yet it looks great on higher settings as well), there's a lot of maps plus additional challenges, and more.
For $2.50, easily a great buy! I've had it since it came out at original price. It's also on the 360 now, but not for that price.