I don't know. The different STB's that I've used from different satellite providers would be a strong influencer for me. I used somebody's DirectTV system last year, and it was terrible. Everything about it was slow and laggy. Obviously I'd want to sample
some more models before making a final decision, but that was a pretty bad first impression.
There are STB's that aren't slow and laggy? Seems like all I've used are slow and laggy... cable or satellite (Charter, Cox, DirectTV, even a couple different models of Sky+ boxes).
Yet another reason why I don't subscribe to cable or satellite... my OTA DTV receiver (I have an old SD TV) is nice and snappy (although nowhere near as full of useless features as some cable/satellite STBs).
On the subject of MediaRoom: IIRC, AT&T's Uverse service uses MediaRoom on their STBs... don't have any experience with that service, though; it's not available in my area yet.