BLUF: The Logitech MX Master is a timeless classic. A big, heavy, solid mouse with reliable buttons, a switchable scroll wheel (clicky or smooth), works with three computers, supports Bluetooth and the Unifying receiver. May be too big for some hands.
Intro: I've been using the original Logitech MX Master mouse for about two years now - it was pricey, but so, so worth it. I've used the MX Master with Windows and macOS, MacBooks, Dells, Bluetooth and Unifying receiver - the MX Master has been unstoppable. After two years of daily use (often 10-12 hours per day) my original MX Master is still going strong. When I saw that Logitech was coming out with a new version, I had to try it out!
Bluetooth pairing/connection: This is the most noticeable improvement over the original MX Master for me. The original had some kind of a strange issue with macOS, where you'd have to re-pair the mouse a couple of times with the computer to get it to connect reliably...the mouse would actually show up more than once in the macOS list of Bluetooth devices. The MX Master 2s does not seem to have this issue - it paired quickly, and stays paired reliably. Bluetooth range is good enough working reliably from 10 feet away (as far away as I could see the pointer moving on my laptop screen). The 2s no longer has a separate "Connect" button for reconnecting to Bluetooth hosts; one simply long-presses the device selector button now. Since this is an activity I typically do once or twice a year, not an issue for me.
Mouse pointer movement: This is what the mouse is for, right? The 2s appears to be, if anything, a little smoother than the original MX Master. While I usually use a mousepad (mainly to keep the bottom of my mice clean), the 2s works smoothly on every surface in my office, from the carpet on the floor to the metal cabinets to my window (yes, it works on clear glass!). Pointer movement seems to be a little smoother than the original MX, which was already excellent; Logitech has changed the surface sensor design on the 2s, as visible in my photo of the bottom of the two mice below. The MX Master touches the table in four places, with harder plastic sliding surfaces along those points; it slides smoothly on most surfaces (even glass).
Button layout/tactile response: The buttons on the 2s are *identical* to the buttons on the original MX Master. I can't feel any differences between them - if nothing else, since my original MX Master has been in use for two years, it's a credit to how durable this mouse design is. The top left/right buttons have a very light feel, little resistance to being pressed, with smooth travel and no sense that things are binding when you press them. The "thumb" button - the one you push down with your thumb, built into the thumb rest - is harder to press, and has very short travel. This is probably intended to minimize accidental button presses, but it will definitely tire your thumb out. The "forward/back" buttons are honestly too small for my liking and too close together (I have big fingers), so I don't use them as often as I used the ones on older mice. That said, the travel on them is smooth, and the action is light enough that your thumb doesn't get tired pressing them.
Scroll wheels: The top scroll wheel can be configured to "click" as it's turned, or to spin freely. In free-spinning mode, the wheel is nicely weighted and will spin for a few seconds on its own without binding or wobbling. The top scroll wheel feels a little better on the 2s than on my original MX Master. I've never really been able to make good use of the side scroll wheel...just couldn't figure out what the point of it was; it can be customized to perform many functions, but I usually wind up disabling it because I trigger it inadvertantly.
Software support: To really make good use of Logitech's mice/keyboards/trackpads, you have to install the Logitech Options software on macOS or Windows. The MX Master is no different in this respect - while you will get basic use of the many buttons without extra software, Logitech Options allows you to customize the behavior of all of the buttons and scroll wheels on the mouse. The current version of Logitech Options works smoothly and transparently.
Comfort: This is a tricky issue - mouse comfort is a very personal thing. The MX Master is *big* - it's the biggest mouse I've ever used, by a wide margin. It's taller, wider and longer than any other mouse I own (and I own about 20, going back to the Macintosh SE). I have fairly large hands, so I find the MX Master to be comfortable to move, click and use all day long; I've let my co-workers try it, and some have said it gave them hand cramps due to its size. Best advice I can give here is try it in the store before you buy it. Because it slides very smoothly, you don't really need to *grip* it - it's more like guiding it as it glides across your desk. A fabric mousepad is recommended for smoothest mousing.
Battery Life: The MX Master has a built-in Lithium-Ion battery, which recharges using a standard Micro-USB cable. Takes 2-3 hours to charge when fully depleted; a charge seems to last 2-3 months for me. There's a 3-LED battery level indicator on the thumb-side.
Conclusion: The MX Master is a great mouse, *if* the size works for you. While the side scroll wheel seems gimmicky, the other mouse controls are solid, smooth and durable. It's highly recommended by many review sites and individual reviews, and my experience with the original MX Master has been great.