I've always used normal mice, and found them uncomfortable, so I was curious about the idea of a vertical mouse. I was considering getting a (much more expensive) Logitech one, but when I saw this, I thought it would be a cheap way to try out the concept before potentially upgrading. After a couple of days getting used to it, though, I don't think I'll bother "upgrading" - this one is as good as I need it to be. It's comfortable, light, and the forward and back buttons are conveniently located right by the thumb.
If you find it uncomfortable at first, you might be gripping it too hard. I find it only needs a very light touch if you rest your hand in the correct position, but it took me a couple of days to get used to the feeling compared to a regular mouse. Also, the cable arrives tightly coiled, so initially it drags the mouse a little, but after a few days of use it straightens out and this problem goes away.