Strong recommendation: do not buy this, it has a design flaw and will fail very quickly (in less than a year) on you.
Feature-wise, this is a fantastic device, and I would use one on every system I work with if it didn't suffer from a fundamental flaw. I am now using my third M570 after Logitech shipped me two replacement devices when the first two both had the same issue, that the left mouse button starts double-clicking or triple-clicking when you want to single click, or not even single clicking when you want it to. The one I am currently using suffers this same problem. I also got one of these devices for my wife, and hers now suffers the exact same issue. Search the web and you'll find the same results. The only reason this device gets high ratings is because those users haven't encountered the flaw yet (when it's brand new, it's absolutely fantastic).
I'm very angry with Logitech about this, because this is a widespread problem that they seem unable to properly fix. Look around the web, you'll find plenty of people complaining about this issue. Whatever you do, don't buy this trackball.
Unfortunately, I would love to recommend an alternative, but I haven't found one yet. The Microsoft trackball devices from years ago were fantastic, however they have also been out of production for years now. Such a shame, because once you use a trackball, moving back to a mouse seems like a step in the wrong direction.