Could've been very very good. You feel flat in so many ways, though - the birds fly much faster than the bullets, so you have to allow a huge amount of deflection meaning that after they get past 60% of the screen they are impossible to hit.
Ammo runs out much too fast, and it's too expensive - the few seconds with no bullets could cost you half your HP.
You can't heal your 'base' either, so one bad level cuts down your chances of progressing hugely.
Overall - yeah, it's too damn hard.