Passable.
It's alright, but after I get the last gun, why would I even bother with the earlier versions? It makes weapon selection a little moot.
Also, while there is an escalation of enemies, there isn't much in the way of options of fighting them off. There are only three guns, and once you get the last one you just bash away at your mouse key and take them out, spending your money only on repairs since it doesn't have a point after that.
What I'm trying to say is that this game lacks options for the players. It's okay for killing a minute or two, but beyond that, it doesn't offer anything else.
If you're going to spend the time and care to create a game like this, try to put in more than just smooth graphics. This is a game, after all. It doesn't have to be a blockbuster smash hit, but it would be nice if there were more options to keep the interest of the player.
Good luck on your future endeavors.