I've had an Evoluent Vertical Mouse 4 for a number of years and I find it really comfortable to use and it has got rid of the pains I was getting up my arm from using a conventional mouse. It is now getting a bit tatty, with some of the silvering having worn off and the black parts have got rough.
So I bought this new one (Vertical Mouse C) which is even better than the 4. It has an even more comfortable shape. I like the bigger lip for your little finger, so it doesn't drag on the desk any more. The materials seem better too, so hopefully won't wear in the same way my C did.
I keep the old one at work and still use it. Having the new one means I don't have to keep packing it up to carry it to and from work.