Good but...
Although I love tower defense games this wasn't to high on my list. There is one huge bug that ruins the game. Just hold spacebar as your killing off the wave and the wave number just keeps rising. I got to lvl 261.
Anway, it has pretty good gameplay but when you're upgrading a tower, it shouldn't de-select the tower after clicking upgrade. That just makes it annoying when you're trying to strategize. Also, not being able to sell the tower is a major drawback.
The final cannon which costs 3000 is a real let down. Despite it doing hardly any DoT due to its crappy fire rate, it costs so much to upgrade.
Overall I thought it was fairly decent but could use improvements.