So, let me get this straight. It boosts wifi, enables wireless printing, wireless music playing and is small enough to fit into your pocket?
Yep. About the same size as the MagSafe adapter. It doesn't just boost WiFi, it can be your only wireless router. http://www.apple.com/airportexpress/
There are no limitations to these features is there? Like, for example, it can only boost from certain wireless routers or it can only print from certain printers? Just checking... I'd hate to buy it and then find out I need to buy even more to use it fully.
Plug in any USB printer and you're set to go. It can extend an existing wireless network, or plug an ethernet cable in from any router and it can create a wireless network. It's my only wireless router, creating a wireless network from my cable modem. Any Mac or PC you choose can join the network.
Your left back pocket. Save a few dollars: http://store.apple.com/us/product/FB321LL/A?mco=MTY3ODQ5OTY
Well hold on there, the APE is designed to extend WiFi networks from Apple products. There are several "standards" for doing stuff like that and they aren't all compatible. The APE may not be able to extend your network if you don't have an Apple router. And even when it is compatible, keep in mind there are tradeoffs like slowing down the network speed. For me, I use my APE purely for the AirTunes functionality. That's worth it alone.
Using the AirPort Admin Utility to create a WDS network with multiple base stations AirPort Express: What is client mode? AirPort Express: How to join an existing wireless network in client mode
Yes, this is true. I bought my Express thinking that it would extend my home network (I use a linksis n-router) but it simply won't work. It'll only extend if you have another Apple router. Still, I'm gonna keep mind for whenever I decide to travel.