I can't say enough good things about my Sonos music system. If you want to play MP3s in your house off a file server and you have more than one room, take a look.
Sonos just got a little better today thanks to the new iPhone controller app. Free app from Sonos lets you control the music queues in all your zones. Works great. Sonos' wireless controller is a big selling point for the system, I'm surprised they developed an iPhone app as an alternative.
Setup was easy as with all Sonos network configuration. The only real caveat is your iPhone has to be on Wifi on the same local network as the Sonos system. It doesn't work via Edge, 3G, or Internet.