I've been running a Dell Studio Hybrid for 6 months. They're assembled from laptop parts so they're very low power. It's completely silent. It has a fan but I've never heard it. It's about the size of a mac mini and starts about $100 less. Looks very inconspicuous in the living room - more like stereo equipment than a computer.
I couldn't compare speed between it and the mac mini, but mine is running windows 7 (because I couldn't get Ubuntu to send sound through the HDMI to the TV) and 2 instances of Ubuntu simultaneously under Sun's Virtualbox and I've never had a speed problem except some sketchiness when running the blu-ray dvd player (regular dvd is fine, Hulu is fine).
It comes with a wireless keyboard and mouse that has a pretty good range - I use it from across the living room.