I've been using a mackie DL1608/iPad for a few months now. It's brilliant. I love that you can save EQs, whole channel strips, gates, compressors, whole mixes, it's just so easy. The wireless mixing makes sound checks a breeze. And the Logic style EQs (and 31 band graphic on main outs & every aux) offer way more tone shaping than the 3 band EQ found on my analogue mixer.
If an iPad were to run out of juice or crash this mixer simply stays with it's current setting so the show still goes on till you get the iPad running again. I always have 2 iPads on stage anyhow, one in the mixer and one running wirelessly near me, and I charge that one between sets. Remember, the iPad is only a control interface, all the computing is done inside the mixer.
I've done lots of gigs where other musicians have used their iPhone/iPad to mix their own IEMs/monitors and they all love it too.
The 1608 is going pretty cheap thesedays if you get the 30 pin model for iPad 2 & 3. (There is a newer, more expensive, lightning version too) Yes, you need to factor in an iPad too, but what you get for your money is amazing. The (free) app is worth downloading to see for yourself how intuative it is. I've engineered some recent gigs on Presonus desks and I personally prefer the Mackie app over the Presonus app. Software updates come via the iPad's App Store updates which automatically run a firmware update on the mixer next time you dock the iPad.
If only there was a 24 channel version I'd have one in a heartbeat. I'm thinking about the 32 channel one for the future though.