I've toyed with it a bit. The three biggest problems are probably the inability to link with libs that were compiled with MSVC (they are working on being able to make and consume libs that are MSVC compatible, but it isn't there yet), problems parsing Windows based source code (i.e. things knowingly or unknowingly relying on MSVC only extensions), and problems with some of the MS STL headers, particularly in 2012 type_traits and some non-conformant macro expansion behavior in MSVC that Clang hasn't emulated (so parsing headers using macros that rely on said expansion behavior fails). I have a patch proposed for the type_traits issues and am waiting for some feedback from Chandler on the patch. It has already been reviewed/signed off on by two others, so hopefully acceptance is close. I believe there are patches for the macro issue the floating around, but they haven't been committed since when one was it broke building gtest :(
