I use mplayer all the time for certain things. I _love_ mplayer and _admire_ the skill of their core team (these guys really know their math and their assembler, and are more security conscious than 99% of the coders out there), their focus has never been on the great user interface experience (they state that as a different skill set IIRC).
Obvious to anyone using SS products, the user interface experience is fantastic, as well as at least good enough on whatever blackmagic needs to happen at the ugly bit/data struture level. Note they are definitely aware of mplayer/ffmpeg as they use it for clonedvd mobile!
So I think the best HTPC user experience would be a standalone SS player. Now, if it was open source and had an LGPL compatible license I'm confident in time aspects of it would get into mplayer.
Note that for a long time mplayer has been able to demux and decode decrypted HD discs. I think it is the only current option for native OSX/Linux playback, in fact.