I'm a big fan of the WindowsXP Media Centre and have been using it for about 18 months (although at the moment my main machine is in the hold of a ship traveling from Brisbane to Seattle so I'm just accessing content that we backed up to a Maxtor from a new desktop machine).
Some people are put off MCE because they think it's complex or expensive (actually neither of those are particularly true) or because it doesn't run on their platform.
Luckily there are lots of options for people who want to try alternatives (though if they're Windows users I recommend they at least get to see a demo of MCE - especially the new Vista edition - before committing) and Eirik just sent me a pretty thorough listing. There's a whole bunch of alternatives on their that I'd not seen before (and pretty good reviews of a lot of them)
If you're new to the platform (or curious what the state of the art is from the commercial vendors and open source community) it's a very good starting point.