Only problem with those is that you don't get all the benefits of a true dual (or triple) head setup. Windows will maximize to span both screens and your taskbar (if you have your taskbar on the external display) will be stretched across all screens. Also, any dialog that usually pops up in the center of the screen (like the login dialog or many message boxes) will be split down the middle across the two screens (not as much of a problem if you have three, though).
So, yes, the Matrox systems look cool, but they're actually pretty painful to use.