You like Geometry Wars? I like Geometry Wars.
It's a shame though that Geometry wars is only on the iPad and not on the iPhone and iPod Touch. Too bad... It actually is on the iPhone. Oh wait, we've got something just as good if not better! It's called Critical Wave, and it rivals Geometry Wars greatly in both visual brilliance and heart pounding, tension filled, twin stick shooter action.
The main difference between Critical Wave and Geometry wars is that CW has stages, upgrades, and massive evolving bosses. Oh, and it's also got regenerating health, so one hit does not quite kill. In fact, I'd go as far as to say that CW is probably the best twin stick shooter in the app store, excluding Geometry Wars of course.