People are still using Visual Studio as far as I know and Visual Studio isn't supporting C99, which makes C99 non-portable as long as Visual Studio is the default compiler for Windows and Windows beeing a big player.
Is windows a big player for C development? If you want to code for windows there are better options. If you want to write C there are better options than VS.
I like PellesC for playing around on my own projects, but have wondered if there's anyone using it for serious development.