If you haven't already upgraded you really want to avoid the iOS 6 update. There are many issues effecting musicians. I'll try to chronicle them here. Please comment with any you've experienced, including your hardware, and we'll get a big list together.
Rheyne tweeted recently about his Ad Hoc WiFi network completely dropping all of his devices right in the middle of this week's jam. I've heard this from several people now, with WiFi just dropping on them. Unfortunately Apple will not let you downgrade iOS firmware versions, so he's stuck with this hanging over his head the next time he gets set up.
Additionally some apps have just stopped working! Mark Mosher posted recently that PPG WaveGenerator and SynthStation just crap out at the loading screen. Wolfgang Palm was contacted and is addressing the issue for WaveGenerator. No one cared enough to contact Akai.
Regular contributor pantsofdeath wrote in with his own woes with Stochastik drum machine. He loves this app, but it just will not load correctly for him now that he's upgraded.
I'm personally disappointed to find Amen Break Generator isn't running on my iPhone 5, but I don't know if that is an iPhone 5 or an iOS 6 issue. If you're a developer trying to figure out if your app is going to run on the new Xcode, on the new iOS and on the new hardware I've got you covered with my professional beta testing services!