Ok, not bad! The gameplay was pretty original, the gradual difficulty increase seemed fair. Nice, but I have some pointers.
At some point the game becomes pretty 'bullet hell-y'. This is ok, but makes one problem with your design apparent. There is no indication when ufo reaches maximum or minimum altitude. This means that a bunch of times I got in a situation when I tried to escape upwards only to be stopped by an invisible wall (and eat a bunch of bullets as a result).
Presentation could use some work. The graphics are ok, but the droning sound of the ufo was a bit annoying. Maybe some music would alleviate this?
Keep up the good work!