These are built from the current development sources (Git master branch).
To set up file type associations and AutoPlay handlers, use mpv-install.bat.
To play content from various media streaming sites, place youtube-dl.exe in the same directory as mpv. I recommend updating it together with mpv. You might want to use this with a browser extension that can open URLs with external applications (Firefox, Chrome).
If you notice anything wrong with these builds, complain to lachs0r on Freenode or report the problem on the issue tracker.
Be advised that the SMPlayer frontend is not supported.
Fixing SMPlayer would amount to a rewrite, which is not very likely to happen. The problem is that mpv has diverged so much from the other members of the MPlayer family that it would be difficult to offer the same level of support for all of them in the same frontend.