It's a bit like the Sonos system but has major disadvantages (as you'd expect for the price):
1. Sonos doesn't need your PC / iPhone connected to a dedicated wireless network to remove chances of interference / dropouts from other traffic on the wireless network - the Sonos system connects into your main network via a wired connection and then streams over its own MESH network. This makes streaming more stable, without the disadvantages of having to disconnect your PC from the streaming network (and therefore stopping music playback) if you want to browse the internet.
2. Sonos reads data from anything on your network - NAS devices, PCs, Macs and now iPhones, iPod Touches etc. etc. With the Airport, without the use of specialist software, you can only stream direct from iTunes / iPhone music app etc.
3. Sonos has a number of other features built in, like streaming from Spotify, Napster etc., internet radio and your playlists can be seamless between these sources. You can set things like alarms, sleep timers etc. on a Sonos system.
If all you want to do is stream music from iTunes on a dedicated PC, then Airport Express does everything you need. But the Sonos system goes that much further. I used to use Airport Expresses, but I moved to Sonos when they had an offer on, and it is much, much better. But then it does cost a lot more, so this shouldn't be too surprising!