It comes in a pretty small box (hand included to give a sense of scale):
The Apple packaging unwraps in the usual folding candy-box manner:
And for something that provides wireless access at 802.11n speeds, it is tiny!
To install it, you just plug the thing into an electrical outlet, hook up an ethernet cable from your router, and run a simple utility on the Mac or PC. If you already have a wireless network, the utility will detect that and offer to configure the Airport as an extension of that network, or function as a new 5Ghz only 802.11n network. It will also easily learn the wired portion of the network configuration and the Airport Express can coexist with another wireless router. So, I basically just plugged it in, answered a few questions, and it was up and running.
The main feature I like about it (other than acting as an 802.11n wireless access point) is it also has a headphone / mini digital audio (fiber) connector, so I can connect it directly to my stereo and pipe iTunes music to it ! Once it's configured, iTunes has an added button on the bottom of the screen to select where the music is to be played - either on the Mac or on the Airport, or both. So, regardless of where my laptop is, I can have it piping music wirelessly to my stereo.
It also has a USB port for adding wireless printing support to a printer, but I haven't tried that feature and doubt I will.
All this for $99.