I've tried quite a few ergo mice over the years: the Evoluent VerticalMouse 3 Wireless - Right Handed
, Ceratech Accuratus Upright Mouse - Mouse - optical - wired - USB - silver, soft black
, the WOW PEN.JOY BLACK
and 3M Optical USB Ergonomic Mouse, Wired, 3 Buttons, Large
I got my first wired DXT a couple of years ago. It took a little getting used to because - like all ergo-mice - its shape is markedly different to run-of-the-mill mice.
But, once acclimatised, I found it to be the mouse I'd always been looking for, in that it prevented my wrist/arm-ache after long sessions with the computer (I'm a software engineer). The mouse is shaped and designed so that there is no difference in the mouse if used with the left or right hands. After a couple of months use I became ambidextrous with it and have even switched to preferring my left hand to my right (I am left-handed but always used the mouse in my right hand previously).
This wireless version is the same size/shaped mouse as the wired - without the wire! As such its the natural successor to my first model and I'm finding it just as excellent. When switching from left to right hand the wire could - if not cautious - become entangled with other things on the desk; cups of coffee being a particular hazard.
All the features of the previous model
(my review of it is at http://www.amazon.co.uk/review/R2JKG1B3MXQHW4/ref=cm_cr_pr_perm?ie=UTF8&ASIN=B003ZK73WK&linkCode=&nodeID=&tag=) are present: the button that switches the mouse from left to right handed, the button that cycles through the mouse sensitivity (500 - 2000 dpi in steps of 500), the nice soft buttons and the smooth mousewheel.
When you pick it up there is no doubt it is a quality product. The casing is plastic, the base is metal and screwed to the case (with tiny screws) not clipped. If feels solid and well made.
I'm absolutely loving it and would recommend it to anyone in the market for such a device.
Update (5-Jul-2015 - after 3 years solid use): Still got this mouse, it still works and still use it every day (I work in IT). It's battered, bruised and the anti-slip coating is wearing away where I grip it. I've de-gunked the mouse wheel several times and dropped it more than once. It works as well as first day I got it.
On another note, i've developed a strategy (to cope with the long hours at the keyboard) where I use this, a 'normal' mouse and, on occasion, a track pad. I find that by using them in a round-robin fashion and switching hands (it really does just take practice) I can stave off any aches and pains before they get a chance to set in. I wouldn't mention it except that I did (mention it) to a colleague, he started doing the same thing and has seen good results too. Your mileage may vary - but it might be something to consider if you're having problems.
Update (28-Apr-2017): Still got the mouse, still using it and it still works a treat. The exterior coating is understandably worn but i'm still getting flawless operation out of it. The battery still lasts as long as it ever did and the buttons maintain their soft and responsive feel. Over the last year i've been spending long, long hours (14/day) at the keyboard. Coupled with an egonomic keyboard, a streamlined round-robin system of alternating this mouse between hands and using it in tandem with a regular mouse (the track-pad mentioned above has been dropped!), I've managed to work these ridiculous hours without problems in my wrists/arms.