Make sure your displayport to DVI adapters are active types (have a USB port).

As for the Matrox boxes, they're limiting in that you have to use the same resolutions per screen, in the same layout. So you can't do say, 2x 1680x1050 in portrait plus 1x 1920x1200 in landscape. On top of that, the TH2Go has a maximum res of 1920x1080 @ 50Hz per screen. A. Not many screens at that res would support 50Hz, and B. anything above that (2048x1152 or 2560x1600 per screen) would not work.

The advantage the Matrox boxes have is they can be used on any graphics card with the correct display output - great if you want to expand your laptops screen real estate. But that's about it.
The Radeons with their capability of running more than 2 screens natively, as well as in Eyefinity mode is just superior in every other way.