I've had a Media Center PC for just over a year now, this is running Windows XP Media Center Edition 2004 (thanks to the Windows XP SP2 update), I initially intended to use the Media Center PC as both a Media Center and a general purpose PC, during the last year however I've barely use the device as a real "PC" - the box has been used as storage for photos, music, digital video (from my Samsung SCD-103 Mini-DV camera), and recorded TV shows - in fact, I needed to add a second hard drive to my HP 864 Media Center PC over the weekend to keep track of all the content.
Is the Media Center PC useful, you bet! - is it made better with the addition of a Portable Media Center ? (you would need to ask the kids that question!, but the basic answer is absolutely, yes).
What could we have done better ? - the one issue I have with the Media Center PC is the size of the box (it's a tower PC) and noise (it's a tower PC) - Intel today announced a new PC form factor called "EPC", or Entertainment PC, this is a super slim, extremely quiet form factor - Take a look at the FIC Web Site to get more information about their implementation of the "EPC".
I'd much prefer to have an "EPC" form factor device slotted into my home entertainment system than a regular (ugly) PC box- I hope that the system can be expanded, the default 160GB drive in the HP 864 Media Center wasn't large enough for all my content, there were power and IDE cables for a 2nd hard drive in the case, but no hanger for the drive, so I have the 2nd drive (a 250GB drive) standing on it's end on the floor of the box, not ideal, but it works.
Also, I'd like to have a box that doesn't boot into the standard Windows XP shell - this device should be a consumer electronics device, hit the power, get directly into the XP Media Center shell.