I have the EVGA UV-16. It's working great for my needs. The monitor on my DVI port is where I do my graphics work. The monitor on the USB adapter is just for email, moving files around, etc.
I have had two problems with it already since yesterday.
First, I can't calibrate the colors. I can get into the calibration utility via the Displays pane but when I make changes nothing happens.
Second, when my MacBook Pro woke up this morning the display was black. A line would occasionally flash on the screen but that was it. I tried rebooting, reconnecting the UV-16, etc. I called EVGA support and they decided that I should RMA it. When I got off the phone with them I reinstalled the driver and it has worked fine all day.
So it isn't perfect. It lags a little bit, can't adjust the colors, and I guess it doesn't always wake up right.

If you need perfect, this isn't the way to go. For tasks that are easy on the video it's a great, inexpensive way to add another monitor.