not to bad
after about 30 levels it got borring.
as it progressed, it didnt become more difficult, it got easier because:
the enemies got bigger.
it still took the same amount of frames to move accross the screen, so they went slower
and you could shoot a bunch at once if they were in the general area, so even if you suck and managed them to get near you, 1 shot would kill them all..
can you loose?