quite good
I really like this game, so the few things I mention here that I don't like aren't too important. Anyway:
The movement of the guy is a bit un-smooth. I.e. when he lands after a jump you cannot start running seamlessly but there is a short delay. Nothing too serious, but would be nicer without.
Also the intra-level animations (the guy running/dieing) really suck after a while when you have to repeat a level for the x-th time and have to wait for the guy to run along the screen preferably followed by an insect. Being able to skip this with Enter would be very nice.
And the sound is maybe somewhat annoying after a while, on the other hand it has to be like that for the retro-feeling, so forget about that.
Other than that I really enjoyed the game, in fact I played it twice and currently a third time. :)