Ive only ported one VERY SMALL XNA game from PC to Mac using MonoGame, but it worked perfectly, no code changes needed, solid 60 fps.
MonoGame basically re-implements almost all of the XNA classes that's why its so easy to port.
Many released games use monogame, look on their new website http://www.monogame.net/showcase
edit: Ive been looking closely at the project and their plans are for a solid 3D support on all platforms first (which is almost there I think) and then later development of a DirectX11 variant of MonoGame for windows desktop (which would be awesome
).