I had a feeling there would be a catch with this product, and there was.
It uses substantial CPU resources - scrolling through a long webpage can hog 80% of the CPU (on a 2009 Core 2 MacBook Pro here) on a 23" 1920x1080 monitor. I expected it to be useless for, say, 3D games, but was hoping it to be at least usable for this common use case. Say goodbye to smooth scrolling - the image is clear but nowhere near as fluid as on the native display.
Not to mention the latest Mac OS X Lion drivers are unfinished betas which leave you without a login screen.
That all said, it *does* work, just slow for anything that involves lots of change on the screen. It loses a star for that, and another star for the drivers (how long as the Lion developer preview been out already?), and price, which is on the high side. Considering they're working with the limits of USB 2, it's not a bad product and can probably find utility in some places where people won't care/notice the slower screen updates, but for me it was a disappointment. Maybe they'll nail it with the USB 3 version.
UPDATE: Tried with a faster i5 based Windows desktop, and performance is much better. From this I gather this product works *much* better with Windows (both performance and driver quality), and having a faster CPU helps too. For this reason I've upgraded it to 4 stars. If you're a Mac user, I'd advise staying away however, at least until they get their act together on the drivers.