Quite terrible.
Even on the lowest settings, my computer was lagging enough so that my mouse crosshairs were flying all over the place. This resulted in me accidentally clicking outside of the game and causing a different screen to load, effectively erasing all progress I had made (was on the 9th) bomb.
This short pause gave me time to think about the game itself, and I realized it wasn't fun enough for me to try and play again, even in a pop-out window where this problem might have been avoided. Overall repetitive and uninspiring. The 20th wave is the same as the 2nd wave, bombs and upgrades do little to change the dynamics on screen, and of course the problem with the lag.
If I can play Crysis on high settings I should be able to play a flash game. My experience with this game was quite terrible.