On the 360, I used the demo downloads only. I have a HTPC for everything else, but I used the media center extender to fire up some music during GoW matches every now and then. The 360 is a bit too crippled to replace a full fledged HTPC for me - it's too limited in setup, requires MS everything, and doesnt work with divx or x264.
On the Wii, I still use the voting channel every few days. It's silly and the questions are often beyond stupid, but every now and then there's some shockers and it's interesting to see how it's different across the nation. There was recently a question: "What worse to have stolen from you, things or ideas? I voted things, and predicted things. Ideas actually edged out things by a few percentage points. Looked at the map of the US (I'm from NYC), and noticed that the Northeast and Nevada (Vegas, lol) were the main places that voted for things. Guess we're all materialistic bastards.
The news channel is interesting, presented very well, but it needs a little more pictures/multimedia before I'm really going to use it.
And I dunno if the Mii Channel counts, but I use it all the time.