i have tried ipad mixing, i do it home when my friends come, and i want to sit with them and relax.
interfaces are very nice.
it can definetely do the work for a gig in a chillout cafe by the beach, specially if you add external sound card (latest native instruments audio 2 and Z1 are ipad compatible, as well as apogee).
i will try to test this setup on big soundsystem when i have chance and equipment.
however i can't imagine a serious gig with this only, maybe if you connect 2 channel output to external mixer, but not standalone, i need precission of analog / physical knobs.
But as a backup or travelling setup is has future.