good
I've played a few others very similar to this, the closest match being Desktop Defense.
A great game, but it does get boring quickly. I stopped after about level 30. The msot annoying are the guys that shoot at your towers, forcing you to upgrade the front row before anything else. I imagine later in the game those shooting guys would make quite a dent in your defenses.
Overall very good, but lack of variety lost you that one point. A few more upgrade options would have been nice as well.