Pros: Super easy to use, doesn't require repetitive movements, faster than a conventional mouse
Cons: Not for everyone, people either love it or hate it. Also lacks a scrolling wheel.
It's not considered a good gaming mouse, although I've never had problems with it. Where it really excels for me is in my work; I'm a newspaper editor who spends a lot of time designing pages in Quark and/or InDesign, and that means literally hundreds--if not thousands--of mouse movements and clicks during a production day. It's well known that these kinds of repetitive movements over long periods of time are harmful. It's also slow and tedious. The Marble Mouse, which stays in a fixed position and navigates with the flick of a finger, reduces the time spent on muse movements to a fraction of what it would otherwise be.
There's an adjustment period for most people who try one of these, but don't use it for five minutes and then throw up your hands. I took to it immediately, and so did one of my friends, but it took another one a few hours to become a convert. Once you do get used to it, you won't want to use anything else.
There's no scrolling wheel, but you can emulate one by clicking and holding the two smaller buttons on each side of the mouse at the same time. These buttons also control the forward and backward functions on most Web browsers; they were added to the mouse about 10 years ago. I've never bothered with them, though. I never used a scroll wheel, and with this mouse, you can move the pointer as fast as you can move fingers to use the extra buttons. Never saw the point, but some people might.
When a product hangs around this long, it's almost always because there's something really good about it, and that's the case here. The only thing I wish would change is for Logitech to produce a wireless version, like they did with the M570 that has the trackball on the side. Other than that, it's perfect.