Well, I broke down and bought one. It feels cheap, and the side-mounted scrollwheel is awful. The motion of the ball itself is pretty good, certainly better than anything I've tried to date (Nasty stuff so far, not tried a Kensington or Logitech).
Meh... Maybe I'm just not suited to Trackballs. I like the Idea, and the coarse control feels a lot better/easier. The Fine control is just not there, but I'll persevere and see what I can get out of it. When it comes to Input devices, I rarely find this board to be wrong, and I can't see you lot being waaaayyyy off-base about Trackballs being great. If I can get the hang of the Fine Control (I am doing some paid PS work at the moment, so the MX518 is still attached to get the Control I need), then I'll invest in a better one.
If anyone else in the UK is interested, then Maplin has them on offer for £20 at them moment. I paid a lot less than that, but I had Gift Vouchers and Staff Discount to use.
http://www.maplin.co.uk/Module.aspx?ModuleNo=227879