I'm currently in the XBMC camp myself, I started with a modded Xbox (the original) back in the day, then upgraded to a gen 1 Apple TV hacked to run XBMC. I've tried a whole bunch of different media playback devices (Popcorn Hour, WDTV, Android and a few others) and always find myself back running XBMC. The first Mac Mini with HDMI somewhat finished that search and I've been using a Mini ever since...
Over the years their have been minor compatibility issues with different encoding formats or or strangeness caused by switching inputs on my receiver causing the Mini to do odd things with resolution or whatnot. This (coupled with me being a tinkerer) has seen me using the nightly builds of XBMC up until the latest stable release which now appears to actually be stable and things are working well.
I've also grabbed an android based little hardware thing that runs XBMC for the bedroom which gives me pretty much everything I know and love (even if the remote sucks balls).
I'm not really using any of the fancy library functions, rather just looking at a list of files (which it neatly overlays with metadata like album art and episode information). The library usually ends up a little messy with duplicates for no apparent reason. Also the issue of moving between devices means I don't know that is marked as watched (or not) on the other players in the house.
At this point it's also worth mentioning that all my media is stored on a NAS device with all devices connected via gigabit ethernet.
I've tried running Plex Server directly on my NAS directly as well as on a Mac Mini, then using my Samsung smart tv's Plex app for playback or even the PS3. In either case the experience was less than good.
Transcoding tends to kill the fast forward/rewind functions and even straight playback is a little squiffy when skipping through a video. That said, it does look pretty and does bring some extra integrations with things like YouTube (although I don't use them too much they are nice to haves). Library management means that all devices would be in sync (although this is more a theory for me since I've only tried it on a single device).
I'm always on the look out for a new media player and keep reading about people using Plex. People seem to want to have it's babies and based on my poor experience I can't figure out why??! Is it any better directly running on a Mac Mini with is doing all playback duties rather than as a server?
So what do you run, on what hardware and why?
I'd love to have a central server with playback on a simple, low powered piece of hardware (the same little thing on different TV's would be nice).
For those who aren't familiar: