I'm generally not very good at shoot-em-ups (Ikaruga somehow excepted). I tend to die, and die often. So playing Who's That Flying today has been a refreshing change for me!
It's a shooter (or shmup) where you're invincible, and like superhero games you may have played featuring this hook instead of worrying about your own ass being handed to you, you have to worry about the city around you instead.
This subtly transforms the game from a fight for survival into a "coverage" game, as you tear into enemies with your bare hands, free from the worries of personal safety, while zipping around the screen ensuring none of them get past you, as let too many critters into the city and they'll eat the place up.
It's a fairly basic game, but also a pretty one, the stark 2D art style reminiscent of Dan Paladin's work with The Behemoth on stuff like Alien Hominid. You can grab it for $6 on Steam, where it's recently been released, or as a PSP Mini if you're a Sony console owner.