I started off with using the XBMC Media Center on a spare Pentium IV machine running Windows 7. The XBox Media Center was originally designed to run on the first XBox, but is now a multiplatform open source (GPL) HTPC platform, with a high degree of customisability and flexibility. The XBMC Media Center uses plugins written in Python, and these plugins can extend the functionality quite a bit, and because XBMX comes with lots of codecs built-in, there's no need to download them separately.
While I really liked XBMC, I also found it a little too advanced for my taste. It can take quite a lot of work to add the functionality you need, and since it's in constant flux, things might break. Still, the extensibility of XBMC is surely an advantage for many of you, but I personally prefer a more turn-key solution for my HTPC.
I since replaced the spare Pentium IV machine with a machine built specifically for HTPC computing: the silent Atom 330-based machine also used in my hackintosh guide. For this machine, I ditched XBMC in favour of Boxee. Technically - you're not really ditching XMBC, as Boxee is a "distribution" of XBMC, but in a more turn-key fashion: lots of functionality comes out-of-the-box, such as downloadable subtitles, support for downloading metadata for TV shows/movies/etc., and so on. Boxee is big on the whole social media thing, integrating them into the application. This aspect of Boxee, to put it nicely, can bugger right off. I just gets in the way and serves no real purpose - for me, at least.
The interface is easy to use, and underneath it inherits all the advanced functionality of XBMC, so you can still get your geek on. I kind of see XBMC-plain as the Debian of the HTPC world, spawning off several off-shoots that deliver turn-key solutions. Another one of such solutions is Plex.
Unlike Boxee, Plex is Intel Mac only, and has no version available for Linux or Windows. I recently had to remove Windows 7 from my Atom 330 HTPC in order to see if I could get Snow Leopard installed (for a follow up to the hackintosh article). This process sadly failed, leaving me with a vanilla Leopard installation. This was the perfect opportunity to try out Plex.
I came away much impressed. Plex certainly has the best interface of the three; it sports the MediaStream theme which can also be installed on XBMC-plain. It's slick, has lots of different views (from very detailed and useful to quite flashy and annoying), and most importantly, seems to have the better metadata download feature (although, for some odd reason, none of the three can download the metadata for season 5 of Two And A Half Men), and comes with additional goodies like playing the theme music of your TV shows while browsing them.
In the past, I also tried Windows Media Center (all versions) and Front Row, but I found them limited in functionality and often cumbersome to use. There are also various non-XBMC HTPC solutions out there that I've not yet had the opportunity to try, so feel free to tell us what solution you are using, and why.