The market for ergonomic devices has always been small, but there's a surprisingly wide choice of vertical mice out there. Unfortunately, a lot of them don't inspire much confidence; some are sold as vertical but actually end up being just fancy-shaped regular mice. The best in class seem to be Evoluent products, which are really vertical and really ergonomic but also really expensive. I wasn't ready to drop almost £100 on something I might not like, so I steered towards cheaper alternatives.
Some of the best reviews on the net pointed towards this fin-shaped CSL wireless mouse, and it was so inexpensive that it wouldn't feel like a waste if I ended up not liking it. The shape makes a bit more sense when you see the SHARKK-branded version - fins, sharks... I honestly don't know which brand came first or which one is more reliable; SHARKK is a US company whereas CSL seems to be based in Germany, but both have a history of distributing and rebranding products by Asian manufacturers, so the original factory is probably the same Shenzhen hangout.
The most annoying element of this mouse is the fact that it does not use Bluetooth, but rather a bog-standard USB wifi dongle. In recent Macs, USB ports are very few and losing one forever is a Big Deal.
The mouse does not require drivers, which is great news for Mac users. It simply appears as a regular HID with 5 buttons: left, right, middle (under the wheel), and two near the thumb. A sixth button, on top of the fin, is actually the DPI selector, which allows you to change resolution (and hence sensitivity) between three modes: 800, 1200 and 1600 DPI. The selector works fine (just cycles through modes as you click it, with no feedback on screen) but OSX just doesn't "see it" as a regular button, which means you cannot easily re-purpose it once you've set the DPI mode. How often will you need to change mouse sensitivity? An operation so rarely performed doesn't really deserve a large button in such a prime location, but there you go.
It obviously takes a bit of time to adapt to the vertical position. Movements that would rely on the wrist with a trackpad or horizontal mouse, now require better coordination further up the arm. Wrist movements are set on a different axis (up-down, rather than sideways), but to be honest it's probably best to keep them at a minimum, relying on your arm instead.
After a few days of use, the pain seems to have gone, which is great news. However, I'm not totally convinced about this mouse. It does take some conscious effort to keep my hand in the vertical position, being way too easy to absent-mindedly slip back into a traditional posture; the fin shape will not stop you from doing it. That's probably the reason Evoluent models have steeper shapes.
In conclusion, the CSL vertical mouse is great value for money: solid build, works flawlessly, cheap as chips. Is it a great vertical mouse though? I'm not so sure. Will I switch to vertical mice for good? Probably not, at this time and with this mouse, but I will revisit this position in a month or two.