So I'm currently using this at work to add a 3rd monitor as an extended desktop to my other two. I have to say with Windows XP at least the set up is really quite nice and easy. Lots of options for cloning or extending and a great deal of control over which monitor is which.
One interesting thing that is worth mentioning is that you can have up to 6 of these. That means 6 additional monitors can conceivably be added to your current set up should you ever have a desire for such a thing (although I personally would hate it as I'd never be able to find my cursor). Now you can only do one monitor per device you buy. If you DO want to do 6, you'll have to buy 6 of these.
It also allows you to add a VGA port to a device that doesn't have one. Why do I care about this? Well...it sure would be nice to play video files on my Dell Duo (tablet/netbook) on my HDTV. Since it has no HDMI, VGA, or DVI port natively, this would allow that. I'm sure you can think of other devices with this same issue...
An additional point of interest for some - normally when you clone monitors in Windows you have to set both monitors at the same resolution. Further it means you have to go with the resolution of the worst monitor as a max and that mixing monitors of differing aspect ratios will yield weird results. (for instance, if you have a monitor that's capable of 1920x1200 and one that can only do 1280 x 1024, you'll have to run both at 1280x1024 or less). This device allows you to clone monitors...at different resolutions! If you don't need this...you don't care. But if you do, you KNOW what a huge deal this is!
The only complaint I have with the way I'm currently using it is that whenever my computer goes to sleep, whatever is on the 3rd monitor (the one on this device) gets pushed over to one of the other screens. Very minor, but I have so many applications open at once that it'd be nice if they stayed put over there.
However, that said here are some other things worth knowing (for most people these wouldn't be issues I don't think, but just in case it doesn't happen to impact you...)
I originally got it for an embedded project since it would have been a lot nicer to have a USB port than a VGA port + power. It was a nice small form factor for it, but ultimately I didn't end up using it because:
1. It requires a driver to install. This means that every computer you plug it into will need to install this software. That's fine if you have a specific computer you're going to use it on. Not so much for an embedded device that needs to plug into a variety of computers.
2. I don't know if it's a problem with Windows 7 or with the specific computer I was using it with, but the "primary" computer for the embedded device didn't do so well with the USB display device software. The resolution stayed set from the last computer I used it on and as for cloning and extending the video card of the computer recognized it as a 3rd monitor. It did mean I could technically use it, but I didn't have all options available and I wasn't able to get the exact set up I wanted because I couldn't access the software. On the XP computers I didn't have this issue. You may find you also only have access to some options if whatever impacted me shows up for you. (It really may be the Win 7 thing)
3. It's a teeny bit of a power hog. I had a USB hub internal to my device. I should have had about 200 mA available for this, but it exceeded it a few times which wasn't a big deal but I got flicker because of that. Unless you are also using it on an unpowered hub with other devices though, you'll never notice this.
All in all, I actually really like it and think it works pretty well indeed. It didn't work for what I bought it for, but that doesn't mean I don't find it useful.
Update: co-worker of mine got one and says it worked fine for him on Windows 7 computer - either they updated the software in the meantime (it's over a year later at this point) or it was just the computer I was using.