I run a triple monitor setup. (2 Voodoo 3 3000's and a Matrox Mystique 220). 20", 17", and 15" monitors. It's quite useful - as for what it looks like, it just looks like one main Windows 2000 desktop, with two extra screens with your wallpaper or background and a mouse that can move between.
Most apps can be dragged to the other screens (ICQ is one of the major ones that doesn't like to move). Games that run in full screen take your primary monitor. Otherwise, it's great!
Z.