Needs a little bit more tweeking
The crosshairs are hard to see during the later levels, particularlly if you get to about level 15. You should make them a bit more visible.
Futhermore, the beam is too exact even with the upgrades. I don't like how if there is a bunch of baddies in one area how only two of them blow up with a beam that could take all of them out.
Other than that, pretty solid game.