Very good game.
A very well made tower defense. The effects each type of tower had on everything else allows for more than just "buy the biggest tower" style gameplay. The hero is useful until you get to the last few levels of the game, by which point you've got a plan for how to beat the level already. Early on, especially if you screw up placing a few towers, the hero can save your ass.
The music got repetitive about 5 minutes in, and the sounds after about 2 and a half hours. The graphics were good, and I got a chuckle out of using sattelite images for the backgrounds. I really liked the road system, takes the guesswork out of things a bit. The ability to speed up and slow down gameplay was a nice touch as well.
Can't say I have any real gripes about the game, except for the lack of a save feature.