Plex is a complete multimedia player and manager for Mac OS X. Plex uses the XBMC code as a framework for both its GUI and the multimedia player, so it should work just like XBMC and other XMBC-derived players out there. Plex is a multimedia player that was designed to be used with HTPC, or Home Theater PCs. Thus, it has what is called a 10-foot interface, which simply means that it has big buttons that can be seen from the couch.
Plex allows you to manage all the media on your computer, network, optical media, and other sources. It has an extensive support for plug-ins that allow you to stream content from the web. Some of the supported websites are CNN, NBC, Miro, Vimeo, TED, Hulu, NetFlix, and YouTube. They are always adding new sites, though. Plex also allows you to connect to iTunes and iPhoto, or even Picasa, and see your pictures on the big screen. And you can control Plex with your mouse and keyboard or an Apple remote, if you have one.
Plex is an excellent player that works very well. I never had problems with it. In fact, I used to use this app to play HD content when others couldn't handle it. Since there are too many similar apps (XBMC, Boxee, etc), it all boils down to personal preference.