IMO Sonos still trumps all the other solutions out there. SB is good and cheap(er) if it works well, but there are a lot of users that have frequent dropouts. Plus SB works best when its server is running on a computer. Sonos can run off a plain NAS drive.
Yes, Sonos is expensive, but in the last month or so I have had it, it has been an absolute pleasure to use without a single dropout. Also has a free iPhone app that has 100% functionality of its controller. It imports iTunes (and other) playlists as well quite seamlessly. I moved my iTunes library to a NAS drive, and Sonos plays songs and iTunes playlists off of that extremely reliably.
Sonos gets even stronger if implementing multi-zone or whole-house audio. Synching is perfect - more than can be said about SB.
The ONLY thing the SB has on Sonos is that Sonos can't do hig-rez music as of now. I used Airport Express for a year before, but that too suffered several dropouts and needs iTunes to run. And you can't do different songs on more than 1 unit if you wanted to (unless you had them linked to different libraries running on different computers). Also love the access Sonos gives to Rhapsody, Napster and other thousands of internet radio stations out there, including all my local ones too).