Logitech barely touched a formula that already worked, and the two things it did change were exactly the right two things.
The MX Master line has been the default answer to "what mouse should I buy" for serious desk work for the better part of a decade, and the 3S sands off the last rough edges rather than messing with the formula. Same body as the MX Master 3, same thumb wheel, same everything your hand already knows if you've used any of these in five years. The two changes are a quieter click and a sensor bump from 4,000 to 8,000 DPI.
The scroll wheel deserves most of this review, because it's still the reason to own this mouse and nobody frames it that way. MagSpeed reads the speed of your flick and switches between a notched, ratcheted mode for precision and a frictionless free-spin that can cross a thousand-row spreadsheet in about a second, automatically, with no button press and no wrong guesses. I have used this wheel daily for years across two generations and it has never once switched modes when I didn't want it to. There are very few mechanisms in my life I can say that about, including several that cost a hundred times more. When people ask why anyone pays $99.99 for a mouse, I don't mention the DPI. I hand them the mouse and tell them to flick the wheel, and I watch their face do the thing.
“I didn't think I'd notice the quieter click until I used it next to an older MX Master 3.”
The quiet click works as advertised, cutting click noise by about 90 percent per Logitech's numbers, and next to an older MX Master 3 in a silent room the gap is immediate. Video calls no longer feature my left hand as a percussion section. Here's my confession though: I miss the loud click a little. A click that clicks felt like the mouse agreeing with me. The new one is polite, correct, and better for everyone I live and work with, and about once a week I click an old mouse in a drawer like a smoker sniffing a pack.
The 8,000 DPI sensor matters on a 4K or 5K display, where it shows up as smoother tracking rather than speed. On a 1080p monitor you will not feel it. That's the whole story of the spec.
Options+ justifies the install for once. My thumb button is Mission Control on the desktop and a different action inside Photoshop, switching automatically with app focus, and the ten minutes of setup pay rent daily. Battery has held near the rated 70 days in practice, about two months per charge, with a one-minute top-up buying hours.
It remains right-hand-only, sculpted specifically for that grip, no left-handed version, same as it's been since 2015. Left-handed friends: Logitech is still not sorry, apparently.
Owners of an MX Master 3 have a hard upgrade case, the core experience barely moved. Everyone else is buying the closest thing the mouse world has to a solved problem, minus one beloved sound effect.