Very impressive and polished. The whole aesthetic ended up looking really good even though at the base level it's probably not much more than programmer art. The sound effects work really well too.
It's also impressive how many levels, guns, etc, how much content in general there is, and (some of) the levels being cleverly designed at that.
The one complaint I have is that the difficulty on some (maybe like half) of the levels gets unreasonable. I ragequit at level 14.