Most people here won't give a damn, but I think there are at least a few who are HTPC-geeks and might care about this. I'm a huge fan of Plex and have been using it for a couple of years now, but the last few builds of it have just made it run worse and worse.
My HTPC is an old C2D Mac Mini with an SSD, and other than Plex, SMCFancontrol and SEP, it runs nothing else. Older verisons of Plex used to absolutely fly on this machine, and these days the media server component has gotten slower and slower, and even after a completely rebuild of the machine when I got the SSD, as soon as I put Plex on it and had PMS start up automatically it's just run like shit ever since.
The old Media Manager for Plex was this awesome no-nonsense interface here. It worked great and made it easy to find everything, correct matching errors, etc, etc.
Recent builds have the new web-based interface, which looks like this. For something as simple as managing media, I think this is unnecessary. It's slower, runs in Safari, and as a matter of personal preference I just don't like it. I just want to check my files and whatnot - I don't need a big fancy web UI that reminds me that I should sign up for Plex Pass.
It was all well and good up to the last release, because option-click would get you the old no-bullshit media manger. But I found this post here, that states that from 9.8.0 onward the old media manager is gone. And that sucks. :\
No Media Manager Standalone application? - Mac - Plex Forums
So with the software bloat + the fact that my fresh OSX install runs like crap as soon as PMS starts up + dumping the simple media manager, I figured I'd give XBMC another shot. I tried it a year or two ago and while I liked it, I found Plex to be a much better alternative. The new XBMC is, so far, pretty damn great. It runs smooth as hell, plugin and skin installs are a breeze, and other than Netflix being a titanic pain in the balls to get working (seriously, wtf?) everything else has been smooth sailing and my HTPC runs great with it.
Here's the home screen. I removed a bunch of the crap I don't use, but it has Live TV, Music and Photo support out of the box. Weather, clean menus and toggle-able recent additions and it all looks crisp. The only thing it's missing is a little notification asking me why I bothered getting the Bloodrayne movies.
Here's the movie list. I could do without the little HD/SD icons, but I think I can edit the skin to get rid of them. I like my setups minimal.
Control menu while playing:
Recently added movies screen. I'm not sure what the numbers are, I think they're probably ratings.
Recently added TV shows screen:
TV Shows screen with episode/plot info (it scrolls automatically):
A big edge that Plex has is the On Deck feature, which will let you pause a movie on one device and resume it on another (eg living room to iPad), along with a lot of extra device support/Apps that aren't quite there yet with XBMC. I don't really care about all of that and just want a clean interface for my media library and software that runs well. XBMC is pretty damn good though, and the plugin/skin configuration settings are easy to use and work exactly the way they should. Other than the Netflix app being awful to get running, I haven't had a single issue with it.
Both are free, so it's a no-brainer to try them both out and see which you like better for your home setup.
For me, the big edge for XBMC is the ability to keep my downloads in archive format and play them without extracting anything. That saves so much hassle. I'm also going to try out the Live TV stuff next week, got a CableCARD on the way from Time Warner, hopefully XBMC can support it.