Mediaportal is one my list of products to try again in a couple of years (when Win7 becomes too dated), but last time I looked, it was still too immature. One of the guys at work uses it, but is always having to frig about.
Currently, MCE + RP (now no longer developed, but has been open sourced) is the best solution I've come across. But its not perfect - anything can be improved upon. But won't run on anything like an RPi

. Powerwise, at full chat, my MCE comes in at about 40W, but peaks to 60W in the first few seconds of resume from standby, as disks spin up. RPi comes in at less than 5W, although (serious) storage is not included.
I can see no single advantage of xbmc over MCE at any level. So for now, I'm sticking with my ultra reliable MCE (now I've ripped out that shitty mobo

- wanna DG45ID mobo with Core2 Quad CPU? Seems prone to BSOD when resuming from standby if connected to a LAN via onboard RJ45).
One thing with media centres, they should just work, and without frigging about all the time. MCE is kept fully updated as part of the standard MS regime, with the exception of TunerFreeMCE plugin, which needs manual updates

, and RP which is no longer developed

. xmbc relying on so many plugins to work usefully will end up being a mare for auto updates at 4am.
Don't worry, I suspect my RPi won't go to waste, I suspect it will still end up as an extender, possibly not running xbmc, possibly running the web I/F from RP, but that will require a mouse

. And it will still act as a fallback for OOF, as it has before

The web i/f issue with xbmc, further investigation is its the api over http (so the mobile phone apps) thats been removed, presumably as its too large an attack vector
http://wiki.xbmc.org/index.php?title=Web_Server_HTTP_API