My experience with that weird thing fully consents with DPR's review findings:
At first blush, the M-Fn Bar seems a creative solution to adjusting a parameter with a wide range of values - AF Area modes, ISO values, or white balance presets. But it comes with three fatal flaws; it's imprecise, it's easily activated by accident, and it simply appears buggy in its operation.
The M-Fn Bar is a solution in search of a problem.
The first issue is perhaps the most damning. It's far too easy to swipe past your intended selection, necessitating a reverse swipe or some taps to get where you wanted to go. If you disable the swipe functionality and use the bar as two soft-touch buttons, you'll still be accidentally hitting the bar and changing the value all the time. You can enable a two-second 'touch to unlock' ability, or have the dedicated 'Lock' button prevent inputs when you don't want them, but that takes added time and misses the larger issue: In its current state, the M-Fn Bar is a solution in search of a problem.
Almost every staff member that's used the EOS R has ended up disabling the M-Fn Bar altogether. With the possible exception of silently controlling audio levels while recording (heavily cropped) 4K video, the M-Fn Bar basically accomplishes the same thing that a standard rear jog dial could accomplish, only in a less precise, less useful and more time-consuming fashion. It's the last control you'll want to deal with if you're trying to catch a fleeting moment
For me the M-Fn bar was the reason why not to buy R, keep the 5D mk III and wait for the R6 to come.