Not sure about the graphical interface that seemingly has had its last public build made in January 2013... Might work fine, but might also break
. Generally I would guess it should be "OK", though.
As for the actual player that's used inside, the person responsible for the mplayer2 builds for MPlayer OSX Extended (pigoz) is now the maintainer of the OS X side of things at the mplayer2 fork called
mpv, and mpv right now seems to work relatively fine with Mavericks. And even if something specific is found to be broken, a visit to the issue tracker or IRC channel should get it fixed rather quickly
. The only problem with mpv is that it most probably will not work with the graphical interfaces created for mplayer/mplayer2 because of changes and code removal (unless noted otherwise in the readme file of that GUI). It does have on-screen controls added instead, which should take care of the most basic needs related to a graphical user interface. Currently
ChrisK3 is maintaining mpv builds for OS X, and you can use the
project-provided homebrew formula to compile it yourself in case the provided binaries do not work.
Mplayer2 generally has died out and there has been no development over at it at all for the past six months, as most people moved to mpv some months before that. Thus, at this point I do not recommend anyone to use it, on OS X or otherwise.
To sum it up: The GUI and the old mplayer2 binaries from last year might work, but I recommend that you start thinking about testing and later possibly migrating to mpv sooner or later along the road, as mplayer2 has more or less stopped being an active project quite some time ago. With all the constant changes, fixes and additions in the multimedia world, this will lead to noticeable problems sooner or later
.