Yes indeed in fact I have got rid of my ATV3 in favour of using it. Mainly using it for streaming ripped DVDs. I ran an old G4 Imac as a media server for use with the ATV3 but found it somewhat unreliable.
Running the Raspberry PI with OPenelec (a version of XBMC) streaming from a USB hard drive connected to airport extreme router. It's little bit geeky to get going - wouldn't suit those who love Apple's plug and play ease, but not too hard if you have half an hour to spare, can following some internet tutorials and are prepered to open up terminal to set up the SD card.
Menus are not quite as responsive as ATV3 and there a few issues at the moment (no airplay audio) - but the code is being developed all the time. Streaming is perfect no issues at all.
Its cheap too - I used an old 2gb card I had lying around and an old mobile phone power supply. So bought the unit and a case - total cost just under £40 all in. Didn't need wireless as I was able to connect it via ethernet to a nearby airport express which simplified things a lot. Using the official XBMC iphone app to control.
