Darwin : Exactly. It uses about 4mb, abit less, a bit more... Like I said, I'll try it a bit since I got it for free, but I don't have huge expectations as I tend to not use multiple desktop features for too long before I decide it's just one more stuff to think about (ie : organized my windows in groups!). Still, it could be useful if you work on different projects and really want to keep things separate. And this will be the case in the next few months, more than ever.
Deozan : To me, it works better Microsoft's multidesktop PowerToy, in terms of responsiveness, design, options... and maybe better-simpler than most other solutions I tried... But it's just an opinion... As I don't have the time to do a full featured analysis-comparison at this time.
There are a couple of well written and detailed revies on the Web site. These are the ones I read before installing altDesk