Additional Mini Screen, urgent!!

Hi everyone,

I'd like to know if anyone ever used an additional monitor, like those : http://www.thinkgeek.com/computing/usb-gadgets/bfa3/
I wanna know your feelings about it, it's efficiency, pros and cons, etc...
Also, i'd like to know if they can be used as principal display in case of hardware failure, i.e. can they work without drivers installation, right when the computer boots ? Also, do these really not use too much ressources (it's said that there's a built-in video card, but it doesn't make all)?
I'd like to know if it can become a part of my IT technician toolkit, or if it's just another useless gizmo   .

Thank you in advance.