Heh, very nice, Blab! I got to the boss with triple bullets. It raped me in two seconds, though. :O Any chance of adding a checkpoint there?
Edit: Actually, now that I've played it again, it seems that the battle ends automatically two seconds in. Is this intended? Or did you forget to freeze the timeline? : P
I like those ship halves that combine to make a full ship. Pretty good idea there.

One problem with bullet collision, though. Sometimes, if the enemy bullets are too fast, your bullets phase right through them. You'll have to either increase the hitbox size behind your bullet, or do some fancy programming.
Some suggestions:
More Health should probably be available from the get go. Not too important, though.
Perhaps add some more of those little purple guys, to spice things up a little. The battles with the large purple guys can get a little mundane, especially since there are four of them in a row.
Adding to the previous point, maybe add some waves of other enemies in between the large purple ships?