So, I built a little system last night to use one of my multitude of PCs for something vaguely useful - using Media Center 2005 (courtesy of MSDN) . Umm...I can't exactly say my experiences were totally positive and it's not really ready for the end-user market! My first horrible issue was an odd error in MCE 2005 complaining that 'Cannot play video as the files required are not installed' as well as a warning that my video card drivers were not compatible with Media Center - not an especially useful error as it turned out, the real problem was due to my internal graphics sub-system (a Geforce 4 MX on NForce2 Shuttle board) having been set as having 32MB of ram...upping it a bit (basically to anything more than 32MB but I settled on 128MB in the end) solved the problem!
Second problem is the sheer crappness of the built in TV-Guide thingy - I guess my using DVB-T (what we call Freeview in the UK)  has flummoxed it! I have about 4 out of 30 channels available for the guide, the rest have 'no data' - not good and a real show stopper for adoption of Media Center in most of the UK (I shudder to think what satellite viewers would see!). Anyway, apart from that (oh and needing a proper Media Center remote)  it works fairly well.
Some issues do drive me crazy though...I don't know of any way to replace the guide with information from other sources - I know it's possible (for example this site has one for viewers in the Netherlands) . There's a bunch of stuff around like this which lets you play around with the settings - but not supporting a simple method of replacing the Guide is not good!
Other bits are mainly to do with the quality of the video output by the system (totally not MCE's fault this time!) video is pretty dark and changing the output via the driver doesn't make much difference  - my card, the Pinnacle 300i does seem a little on the crappy side (one of the indications is that the upgrade procedure asks for your serial number...unfortunately nowhere in the software does this exist - hint - you can use any number)...Anyway, I'll persevere, any tips on using MCE 2005 with Freeview in the UK are gratefully accepted! (An I'll happily review any Media Centers anyone wants to send me ;-))

UPDATE: Umm...yes...seems you can add channels to the list...they are actually in the guide just not hooked up to the actual channels (no idea why) - hooked them up and I have a guide - still crappy picture quality though :-(