OK I bought one. US$15 locally. It works as advertised, except that using the trigger is a bit awkward.
It works best with the mouse pressed up on the armrest or a part of the body, or else two handed. That's OK though, since it means my arms are no longer tired since my arms are just sitting in my lap.
The only problem though is the one I got is PS/2 only.
I'm thinking of getting a PS/2 keyboard and mouse to USB adapter. See
here.
Anyways, this thing is great for pure surfing, but less useful if you're doing a lot of typing, since for the latter you're going to have your arms on the desk anyway.