I'm a software developer who switched to using trackballs and vertical mice for work several years ago. My daily driver is the Logitech MX Vertical Mouse (but I've used the Evoluent and others). Vertical mice are great, but they introduced the problem of needing to bear the weight of my arm to use them without wrist motions - after a while, I start slacking and twisting my wrist (like paying attention to your posture and then slouching as soon as you stop thinking about it). That's what I'm trying to avoid! This neatly solves that problem by letting me rest my wrist on it, moving with me, so I can easily move the mouse with my arm instead. With my elbow on my arm rest, it's great.
I use the left side with vertical mice and the right side with trackballs and standard mice. If you only use trackballs I wouldn't recommend this, but I find it handy with standard mice.
It glides across my desk pad without resistance but stays still when I need it to - no shifting or anything of that nature.