I generally set that extra key -- it's positioned where your thumb rests on the mouse -- to "Windows navigation," which allows you to quickly navigate between windows/apps you have open on your desktop. You can also set the gesture button to be a "media control" button, but I could only get it to pause and play tracks in Spotify and raise and lower the volume (you slide the mouse forward or back while holding down the gesture button to adjust volume), not skip tracks forward and back. In other words, the gesture controls may or not work fully with all apps.
One of the most important buttons on a mouse is the scroll wheel. And one of the key features on the MX Master is its speed-adaptive scroll wheel that lets you auto-shift from click-to-click to hyper-fast scrolling (you just hold down the wheel), along with a thumbwheel for side-to-side scrolling and page switching, which is great for spreadsheet jockeys.
That scroll wheel and its smooth, precise nature is one of the components that helps justify this mouse's premium price. To bring back the car analogy, the mouse handles more like a luxury vehicle than a standard midrange sedan. It's not a huge difference, but it's noticeable. It just feels good to drive.
It's worth noting, however, that this mouse is for right-handed operators only -- there's no left-handed option. (Logitech says it tried a left-handed mouse a few years ago but it was a major flop; apparently, lefties use their right hand for mice, or opt for the trackpad.)
With so many options it does take some time to figure out what buttons do what, and how you want to best program them for your needs.
Not everybody wants -- or needs -- to spend $100 on a mouse, and the MX Master may be overkill for a lot of people. But if you're a so-called "power user" who's moving between documents, spreadsheets, and any number of content-creation apps, you'll certainly appreciate what this mouse has to offer. It's also a good choice for someone who needs to switch between multiple computers on the fly.
I currently use a trackball with the Windows PC I use at work and anwith an iMac I have at home. While both cost less than the MX Master, my time with it has left me pondering a switch. I don't know if that makes this the best mouse ever, but it's certainly worth considering if you're in the market for a premium mouse.