I have an app that seems to crash on launch on the original iPhone running iOS 3.1.3 when compiled with LLVM 2.0, but runs just fine with LLVM-GCC. I support back to iOS 3.1 so this is fatal. Not sure if there's an interaction between LLVM 2.0 and some specific code I have, but it seems best to avoid LLVM if you need to support iOS 3.x unless you can test thoroughly on legacy devices.
Update: It seems that the issue is with the device hardware rather than iOS version. 1st and 2nd generation iOS devices seems to be affected: Original iPhone, iPhone 3G, and 1st and 2nd gen iPod Touch. I believe this means it's limited to ARMv6 architectures.
Also, running a debug build via Xcode's debugger works fine, while release builds installed via iTunes do not. So it might be an interaction between CPU architecture and optimization level with LLVM 2.0.
But in any case, avoid for now ;)
