[Updated review based on experience of customer that I bought it for]
[Updated further on 14 March 2012 to give experience of using with Windows 7]
After overcoming initial installation problems (see below), the device appeared to work perfectly. However the customer that I bought it for has had repeated problems with the device disconnecting and then refusing to re-connect despite unplugging/replugging the device from its USB lead, rebooting the router, and rebooting the PC. Signal strength varies wildly between good (four bars) and very poor (one bar). Even when the signal strength is good, the device will sometimes fail to connect: it just prompts for the WPA key to be entered again. Other times, the device works perfectly: when I was last with the customer I rebooted the PC five times and each time it connected immediately and stayed up, but the following day she reported that it was up to its old tricks again.
It's talking to a BT Homehub Wireless N router. In an effort to reduce any possible compatibility problems, I've tried restricting the router to Wireless B/G (turning off Wireless N) and to WPA encryption (as opposed to WPA+WPA2). I've also fixed the channel at a different one to the default (auto) value because of a neighbour's router on the original channel that wasn't broadcasting when I originally set up the customer's router and wireless adaptor. All to no avail.
In short, this device is unreliable. I'm about to try to return it for a refund as being defective.
Installing the drivers is far from straightforward. As with so many unbranded devices, the problem is that the instructions bear little resemblence to reality.
The instructions imply that the setup program will auto-run when the CD is inserted. This is not the case: there is no autorun.inf file and no setup.exe in the root directory. Instead there are two folders with very similar and equally plausible names which describe subtle variants of a model number (I forget the precise details now) and there is no model number printed on the device itself. Within each folder are several more for the various operating systems for which there are drivers. Choosing the correct operating system is easy enough, but I had to try each root-level device folder in turn. Sod's law: the first one I tried installed perfectly but I got a "drivers cannot be located" Windows message :-( OK, uninstall that one and try installing the other one instead... Bingo! When the driver installation had finished, I plugged the device in (though there was nothing to tell me to do this...) and it was detected. From then on it was a text book setup: available wireless networks were displayed, I chose the correct one, supplied the WPA key and we were in business.
I have the knowledge and experience to be resourceful and to try things in turn. But I bet many people would have given up. The instructions need to be modified to describe exactly what needs to be done, rather than glossing over some issues.
[experience with Windows 7]
I've used the device with two different Windows 7 PCs and it was very different! The device installed painlessly, finding the drivers using Windows Update rather than needing a CD, and the device seems to work flawlessly: it reliably connects whenever the PC is started from cold or from standby, and keeps the connection fautlessly. This is better than the Netgear device that I used before, which had problems with the add-on package starting and then immediately dying when the PC came out of standby, requiring the device to be unplugged and plugged back in to force it to be recognised, and would occasionally lose connection after that.
So it looks as if my earlier problems were confined to Windows XP and/or the customer's router; on Windows 7 everything seems fine.