It's a decent game, the atmosphere and controls are well done, but there are a few flaws.
The scenery is very flat, so gameplay consists of mostly running left and right shooting at enemies, which aren't very varied. The difficulty could be improved, also, as it gets a little easy toward the end, especially as you can top up your health in the pause screen anytime.
More terrain, more enemies, and different objectives other than 'clear the screen then go stand by the lab or plant between waves'.
But it's still not a bad game, and it's satisfying to play.