Hello Gilles,                                        

  GG> Logitech has several trackballs at their catalog, but
    > none is actually called "Marble Mouse"               

That's what it's called in their (U.S.) on-line catalogue...

http://www.logitech.com/index.cfm/products/details/US/EN,CRID=2150,CONTENTID=50

I should have included that link in my original enquiry.

Most of what people have said so far sounds positive, so I
may buy it anyway.  If it turns out not to work, I'll find
someone to use it.

- Andy.