I've tried those in the past - both in Linux and Windows - and never got used to them.
You obviously don't get the added benefit from an actual multi monitor setup (which I would have a hard time living without now that I've gotten used to two 1680x1050 monitors

) - being able to see additional windows at once - so I felt like I really didn't gain much of anything over simple alt-tab task switching.
But I guess that if you're regularly running a significant amount of applications, it could be a help to group them on different virtual desktops according to task.
Actually, I gotta admit that it can get a bit crowded even with my dual monitor setup. One of them is only a 15.4" laptop screen, so most of the work where I actually have to read or look at something for extended periods of time, takes place on my 22" monitor.
1680x1050 on a 15.4" screen really isn't particularly eye friendly, even with 125% system font size

.
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