Lots of fun. Nice balance between staying safe with low firepower (one finger) or maxing your firepower for less flexibility and a bigger hitbox (five fingers). The ability to switch how many ships/fingers are on the screen at once leads to some interesting and new gameplay, which could really only be done so well on a touchscreen.
I imagine it's pretty tough on a phone-sized screen, but it's awesome on iPad.
I'm happy to pay up for more of this game, but knowing how much I'm going to be paying and how much content is behind that paywall would be really nice to know ahead of time.