The new Xcode 5 developer preview is already working well. I already tested our Filemaker and Xojo (Real Studio) plugins here with it and they compile file here. Already I found & fixed a few bugs found by the improved static analyzer.
If you have access to the WWDC videos, please take the time to watch them. Including the ones from the last years. There are sessions about LLVM which will also be used in next year's Xojo version. So It's good to know the details about that compiler. You find all videos from WWDC conferences on Apple's developer video page.
Of course once we move here to Mac OS X 10.9 as development platform, we may need to quit compiling Real Studio plugins for PPC target. Also at some point we may loose backwards compatibility for Mac OS X 10.4.