I've seen plenty of people complain about the latest Logitech offering - after a bit motion becomes jittery - and lamenting that the previous discontinued version isn't available.
Since I'm already ordering from Scan I can save on delivery if I get a similar looking one by Speedlink (the only trackball Scan have):
Is a new product though and can't find any reviews. Other speedlink products get reasonable reviews though, so I'm thinking I'll probably go for it...