Decent, solid game
This was OK, but there is lot you could improve on in a sequel.
The upgrades did not scale, so you can purchase all of them after the very first mission. And while I did notice a difference, it was slight. The only thing I really noticed was the improvement in accuracy, which actually decreases accuracy with a machinegun, because you need to click exactly on the enemy instead of clicking in the general area and letting the reticle-shake hit it for you.
The weapons weren't differentiable. I couldn't tell a difference between machine guns, and there was no practical difference between the two smaller missiles and the larger two missiles. You could get all of them before level 3, anyway, which took the anticipation of a new weapon out of the game. Plus, if you use the missiles, then you always hit a civilian just off the screen, which takes the fun out of it.
The maps were very long and tedious. I found that it really didn't hurt much just to fly over everything until reaching the objective. I think shorter but much harder would be better.
I liked the diversity of the missions; mixing up rescue, sabotage and capture. Maybe you could add even more diverse missions and let you take along different equipment for each mission.