I'm not really into shoot-em-up games, so maybe this is so fun for me because I don't get killed off right away.  I like this game because it is simple, it's fun to advance through the levels, and I actually like the techno music soundtrack.  The graphics are simple, but colorful and playful.  And the titles at the start of each level are funny.  This isn't sophisticated or anything, but it's always something I like to play when I'm bored, and it can keep me busy for hours.