I liked this game and even wasted a full day on it, just going back to it every once in a while because I'd get burned out trying to beat a particular level.
I have completed everything up to but not including the final level. Insane is right, I can't see how I could win except an unusually lucky series of waves that would be easy to kill but give plenty of UP to spend on upgrades.
As far as tower defense style games go, this was a nice change of pace in that you can actually participate and not leave it ALL up to automation. I found myself, unfortunately, kinda pigeon-holed into using double penetration lasers and a turret from very early on in the game because everything else took too long to fire, wasn't as accurate as I'd have liked and/or didn't hit enough enemies when a good shot went off.
I liked the upgrades and found myself in love with the stun traps (freeze just didn't cut it for me) but after a while, the faster a level progresses, I just found myself *almost* exclusively using the basic turrent and upgrading from there due to time and monetary constraints.
I played well past the point of having every single talent max'd out and still found myself to have trouble on a few of the later levels so kudos for difficulty/replayability. Sadly, the final level, as mentioned earlier, is simply too hard.
I thought I had problems earlier on in the game when the shielded, flying enemies would be super-fast, be tightly packed together in a swarm and friggin' TELEPORTED all at the same time (seriously, most overpowered combo I've seen so far, they didn't even NEED to be "tough" or be able to "split" since I had literally no chance of defending against them, anyway) but the layout for this particular level simply does not have nearly enough room for my taste.
Obviously it is quite doable but I guess I'm just not good enough nor lucky enough to be given a reprieve in this game. Oh well.
Altogether, I found it to be an enjoyable experience but when you bash your head against one level for so long, you can imagine how draining and overly demoralizing it can be.
It's a good game but bloody hell, the difficulty curve is artificially steep and leaving anything up to pure, random luck in a game such as this is a bit too much for some such as myself to handle.
Regardless, I thoroughly hope more quality works are pumped out that can rival this and mechanics tweaked so it is not so overwhelming even on normal mode. =)