I decided to go for a top-of-the-line mouse since I spend so much time with one in my hand. This one is extremely comfortable -- probably the most comfortable one I've ever used. My hand doesn't feel fatigued or sore after hours of use, as happened with most other mice. The thumb ledge is weird to get used to, but it makes for an even more comfortable experience. If you have RSI, tendinitis or just feel pain or discomfort after much mousing, this is an excellent solution.
The tracking is smooth and precise. No complaints on that front at all.
The Bluetooth pairing is excellent. Zero lag and no unresponsiveness after not using the mouse for some seconds or minutes. It responds like a wired mouse. And this is on both Linux and Windows.
The MX Master 2S does have some downsides, but nothing I can't live with:
1. Mousewheel does NOT have side-to-side buttons. I've grown VERY accustomed to these and still miss them. It's hard to believe such a high-end mouse doesn't have them.
2. The thumb wheel, which is supposed to replace the mousewheel's side buttons, works completely differently and is no replacement for them. I used them to switch between virtual desktops. Doing that with the wheel will give you nausea, as the desktops fly by. No matter how much you try to use the thumbwheel to do something you did with buttons before, it won't work out. You're going to end up overshooting and scrolling back no matter what.
3. The two "extra" buttons next to the thumb wheel have a truly awful design. They're shaped like long opposing wedges, and your thumb will naturally lean on the bottom one and hit it with the side of the thumb. The top button requires you to both raise your thumb and retract it significantly in order to press it, on the other hand, so the movements to hit the two buttons are neither symmetric nor parallel nor mirrored -- they're idiosyncratic. And when you try to hit the top button, you're still going to hit the wheel 4 times out of 5, among other things because its larger end segues flush into the thumb wheel. Piss poor design!
4. The button for switching between computers is on the bottom, making it cumbersome to do so. The M720 Triathlon has the button placed very conveniently where your thumb can get to it without even looking.
5. There's no compartment or hole in the mouse to store the USB receiver, making it very easy to lose and even easier to just not bring anywhere.
Even though the Logitech M720 Triathlon is clearly a much more cheaply built mouse, and is far less comfortable to me, it has none of these failings. I really wish Logitech would make an MX Master 3S that addresses points 1-5 above. It would be pretty much the ultimate mouse!
Still, this one is COMFORTABLE. Can't emphasize that enough.