Actually, I quite like media-portal.. to me it's much more customisable then MCE.. My brother is running both atm on his Elonex Artisan..
The real strength is that since it's open source, and being worked on, more and more features are being added..
It's ability to be completely customised helps, being able to specfy external players for all media types is handy, getting MCE into a state to play everything was getting difficult with my Brothers MCE setup, I installed Media_Portal, and used a combination of Zoom Player, and even VLC at times , and it plays absolutely everything we've thrown at it..
The TV interface seems hardware dependant as well, it's a bit sluggish on my cheap DVB-T card, but much slicker on my brothers DVB-T cards (Top end Hauppage)
Hourses for courses, perhaps I'm missing something obvious that MCE has the Media Portal can't do..
Then again customisation just means more setup at times, a double edged sword..