Bought it so I can stream my music from my Macbook to my portable speaker in the kitchen. It works great, and very easy to setup. I've dealt with various wireless routers (D-Link, Netgear, Linksys, etc) and all of them are pretty difficult to setup. Most of them requires you to connect via ethernet first to configure them. For the Airport Express, I connected my Macbook wirelessly, set-up the Airport Express to join my current wifi network, input the encryption key, and voila. Click couple of checkboxes in iTunes, and I'm set to stream my music.
The downside is price. The Airport Extreme cost $100 retail a pop. Can be pretty expensive if you want to wirelessly put music eveywhere in your home. All I need now is a full-featured remote control so I can control iTunes wirelessly, complete with track display and ability to rate tracks. Maybe an iPod with wifi will do this.