I don't use the MSVC toolchain or Windows for that matter, so I don't have a good compass for this, but the impression from the newsletter, dev meetings etc is that clang-cl is constantly being improved. Is the situation still that poor?
I think it's pretty acceptable, if you're writing C. As you can see, C++ support is only partially completed (no exceptions!). But you're right, Clang on Windows gets constantly improved.
The creation of good debugging information (MSVC's /Zi or even /ZI option) is also missing. For a first class citizen, support of the (undocumented) PDB format is a requirement in my opinion.