Brilliant but only slightly innovative.
Okay, so I AM biased because I love a good TD game. And I did beat it after about a half-dozen plays. Level 39 is the hardest, IMHO. Tip: you have to be willing to sell some towers if you feel you need to upgrade or switch out your defenses. Keep an eye on what's coming. And for sure, leave the auto-wave off! Then you'll have time to think and plan.
Okay, about the game: Sweet graphics. Nice variety of enemies and enemy capabilities (man, heal and freeze are bitch). Decent choice of towers, very low gold accumulation -- of course, that's what makes it hard.
In the end, of course, it IS "just" another TD game -- but the difficulty and the reward of the beta invite are what set it apart. Very nice.