APPARENTLY, 2 first versions of Media Center (part of MS Windows XP in Media Center versions) were not good enough as functionalities were there but "quality of video was less than a $100 DVD player" (Here, I only quote the main criticism that has been aimed at 2 first versions)
APPARENTLY, 3rd version (Media Center 2005) will offer better video quality.
The REMOTE is the big thing here.
In summary, if you can grab a Media Center PC for a decent price, go for it.
Some manufacturers do choose to implement their own or partners solutions - other than MS Windows Media Center - which work as well and might be cheaper.