HTPC has been the most common use I've seen. I use my Raspberry Pi to run XBMC. It gets the job done although the menu can be a little sluggish; I've been thinking of upgrading it for awhile.
Just a regular B, actually. I bought it before the B+. The B especially is something of a struggle with its two sole USB ports. I use one for the wifi adapter, and the other to plug in my external hard-drive. I have to control the program entirely via my phone, since I can't plug in a keyboard or an IR adapter.
Yeah the interface for XBMC struggles on a B+, though playback works fine.