I would bet that most people who want to use it also have a similar library. For the thread stuff I just adhered to the pthread interface, and for atomic operations, I grabbed similar functions from the Linux kernel. (If you need code to do a low-level systems thing, chances are the Linux kernel needs it too. Fantastic resource.)
I see this as a standard that's less providing new things, but providing consistent names and interfaces to things many people already do. Personally, I had always seen Pthreads as the defacto thread, mutex and condition variable standard for C. But it makes sense to define one outside of Pthreads for non-POSIX platforms, particularly if you already need to add atomic and thread-local to the standard.
The stuff on our site (see profile) is pretty well documented in the .h files... our next release (hopefully in a week or two) will offer something like MSDN-style overview pages along with detailed function docs.
Really wish there was something like rdoc for C code--and no, Doxygen is fugly and fail.
I see this as a standard that's less providing new things, but providing consistent names and interfaces to things many people already do. Personally, I had always seen Pthreads as the defacto thread, mutex and condition variable standard for C. But it makes sense to define one outside of Pthreads for non-POSIX platforms, particularly if you already need to add atomic and thread-local to the standard.