Pretty good, but unfair at times.
First of all you could probably safely take out the holster/deholster key, as well as the speed key. I ended up keeping the gun out the whole time and running constantly. Your gameplay isn't dependent on these functions, unlike Flashback, so unless you're planning on expanding the gameplay, I wouldn't keep it in.
I like the split screen effect, where you basically get to see what's coming up without having to engage the enemy first. Reminded me a lot of Oddworld in that sense.
I had some problems with the gameplay. First, it seemed like the only way I could get more ammo was to let myself die. I might have missed something, but I didn't see any way to gain more ammo. If there is a way to gain ammo, you should explicitly state it somewhere in the instructions. Second, once you die once, usually the enemy will kill you once or twice more after continuing before they even let you get your gun back up to shoot them. Third, one time I came out of a cave facing the wrong way, and the three guys there killed me before I got turned around.
Work on these and the game will be a lot better. It'd be nice if there was a little more variety to the gameplay, as well.