I've been looking for something like this for a long time, something to carry in my laptop bag when I travel and something to carry in my IT toolkit.
This device has 3 operational modes with different uses. I wouldn't recommend this device unless you need to switch between these modes with the same device, but that feature has good uses.
There is a Client mode. This will allow you to join the device to a wireless network, and bridge it to the ethernet port. This means you can connect a wired device to a wireless network. The downside to this mode is that one must first manually configure an IP address in the subnet of the Dlink default IP (192.168.1.50) so one can get to the configuration, then reset their IP back to dynamic or an IP in the wireless network you're connecting to.
This is useful for when someone moves a wired only computer or printer where there isn't a wire yet, and you need to just get it working for a while until the cabling happens or a more permanant device is ordered. You can also use it to connect your Xbox over at a friend's house. They may have wireless, you may have wired like in my case. Just configure it for their network and plug it into your xbox and bring it over. There's other potential uses for this mode too.
There is a Router mode. This will let you share an internet connection on the wired port with wireless devices. This puts them behind a NAT router as well, with all the usual settings. This is the easiest to setup and understand, just like your typical router but missing wired ports. You can get in on the default dlink wireless network, go to the default IP (192.168.1.50) and then change any settings, setup security, setup a static WAN address etc.
This is useful for when you want to share a dsl modem with a group of laptops, or join a laptop to a wired network but have it behind the NAT firewall, isolating it from incoming access from that network. I really won't use this option much, but as an IT guy, it's helpful to have in my laptop bag a spare router that could be setup for wireless clients in a pinch.
There is a Access Point mode. This will let you bridge a wired network into a wireless network. This bypasses any firewall/router and will share the wired network's DHCP and all that. This works out of the box with no configuration, but I'd recommend connecting to the wired port with a static ip to connect to the 192.168.1.50 and changing the name/setting up security. You don't want to have rogue access points allowing people unsecured access to your wired network usually, even temporarily.
This is the most useful mode for me. You can use this at locations that don't have a wireless. Just plug it in, and then you can connect your laptop and roam around without having to cable your laptop in. You can connect multiple devices as well. This is good for taking a hotel with a poor or no wireless connection but a wired one, and still using your iPhone/Android wifi for example. Most of my clients don't use wireless on the corporate lan, so I can use this to jack in and use my tablet without dragging the cord around.
It comes with a nice zipper case for cables, AC adapter and the device, though it is slightly larger than it needs to be.
It has a USB cord to power the device off the laptop instead of the AC adapter, but it seems like the range is reduced in this mode.
The range isn't as good as a larger wireless N device with larger antenna (internal or external)
There is no antenna jack for an external antenna.
There is a USB port to share with the Shareport utility or use a 3G modem with it, only in router mode.
Each mode switch has it's own config, passwords, defaults and so on. I suggest using a labelmaker to record the important settings on the device for each switch mode if they are changed.
It supports WPS and has a WPS button even, but I didn't test that since I dislike automagic wizards.
The only way I can imagine this being improved is the external antenna jack, a second LAN port for LAN router mode and maybe a battery (ala mifi devices) to run for a few hours without having to find a power cord.