Very, very rewarding and satisfying shooting action. Both music and (especially) sound effects are a total hit, no pun intended.
The physics are good, some weird stuff can happen when lobbing multiple grenades at once, but no complaints otherwise.
XP and rewards are given at just the right rate to keep you playing. The challenge is high, I had some tense moments especially with the defense mode, however there's one gamebreaking weapon that rules above all else, and I don't know if I should say which one, so... I'll leave it up to you to find out!
The only glitch I found was that sometimes my weapon would appear twice, with one fixed in starting position, and that caused the game to freeze after a few shots, either that or making my HUD disappear for some reason. Nothing that a quick refresh can't fix though!
Bottom line, very fun, great coding, awesome work!