Pretty Good
As defending games go, this isn't half bad. The main game is challenging, maybe a bit too much on easy, and there is a free play mode that lets you play around with lots of cash. That second mode really helps with the replay value.
One qualm I have, though, is that you don't seem to gain more money as the waves go up. The enemies get harder to kill, but they are worth the same amount and they are always the same enemies, over and over again. Perhaps, in the next installment, you could add in color variations on the enemies to add more enemies to the army and more value to the kills. As it is, you play through, getting the same amount, until you hit a stalemate around Wave 25, then it all goes downhill from there. It might also help if you could upgrade the units, or if it took a different amount of money to upgrade the different turrets.
Overall, a decent game for the holidays.