Currently got this one on my work laptop. Model M terminal board with internal converter. The only layout changes I made versus a normal 102-key are that RCtrl is is a Windows key, and the four keys along the right side of the numpad are =, -, +, and the normal Enter.
Those have been discussed in some depth in some of the keyboard communities, and the charitable opinion is that they are for a very niche audience that wants to pay for a specific level of configurability without buying new keycaps, and that is willing to sacrifice features that hobbyists like to pay for, including modern design elements, mounting methods, and somehow both standardization and further customizability. Of course, you’re also taking a positive step to support System76, which I can’t complain about.
Basically, though, you’re paying a lot of money for the dream keyboard of one System76 engineer, circa 2019. It’s not “bad” exactly, but it would be understating it to say that it is a quirky product, even among keyboard nerds. It’s also, within that space, a very different product than these 20- to 40-year old classic buckling spring boards.
They use your standard Chinese cherry clone switches. Better than the rubber dome keyboards everyone gets by default, but nothing like a genuine model M. (the other reply linked to a model F which is likely better than the M but I wasn’t aware of that option until now so I’ll have to try it)