I switched from PLEX to XBMC on my HTPC running OpenElec that sits in my a/v cabinet and hooked up to my system. I love PLEX for music and I still use it for that via my iPad and AppleTV. I airplay from my iPad to my AppleTv and it works great and give me, and more importantly, my wife, an easy and slick way to access our music from any room in the house.
For movies, maybe things have changed, but I could never get PLEX to do quite what I want. If I'm buying Blu-Rays, I want the hi-def audio codecs. As far as I know, PLEX doesn't do that... or does it do it now?