Fun and addictive, but quickly repetitive. The difficulty curve is about perfect. The advanced weapons, though, just aren't worth their cost in energy. Some tweaking would be useful.
This game had me going for months. Incredibly fun, insanely difficult to get the impossible badge. Here's a hint, now that I've finally got it: Snap towers, when maxed out, will freeze all units they hit for about 10 secs.