Alright, but some prominent flaws...
I like the style of the game, classic action. I think the crude graphics give it charm too, it fits in with the retro feel.
However, it was quite glitchy. Sometimes if you killed something, it stayed on screen doing whatever it had been doing. It didn't collide with anything, so it wasn't really a big issue for the aliens who damage by contact - they'd just go off the screen. But if it happened with aliens armed with beam weapons, you have essentially invincible enemies constantly shooting at you.
I found another bug when I was crouched to shoot at the crab-like aliens: character was shown shooting, but it just didn't affect the aliens. I didn't get a chance to look into the bug in more depth though, because I already had low health and died. (I encountered these bugs using Firefox 3.0.5 and Flash 10.0.12 on Windows, which I'd say is a pretty common configuration).
Apart from those bugs, I only have one other suggestion - a period of invincibility after being hit. It doesn't have to be long, even half a second, but it's a little ridiculous that you can have reasonably high health, but if you hold forward a bit too long when entering a new screen, you can be killed by the electricity things on the floor in a split second.
Still, it was enjoyable, and has the potential to be a fairly decent retro action game.