I really liked it. True, it would have been nice to place the turrets myself, and the graphics could have been better, but all in all I had a good and gravy time. - And to anyone who thinks that the upgrades are too expensive, they are not. - In fact the 'Invest' function is really cool, and is perfect for multiplying the money that you get from enemies.
Additionally Mic337, I am a 3D artist by trade. If you ever want some sprites let me know, it'd be no skin off of my nose to make you some :)