It was a tad easy I have to say, I beat it on the first go. Very straightforward upgrade options and strategy (basically only three to four towers are ultimately worthwhile).
Still, I do like the idea of a Tower Defense game oriented around points instead of rounds. The Dares and Double Dares make it much more interesting....
Here's another recent Tower Defence clone I've enjoyed:
http://www.onemorelevel.com/game/ant_buster