I use the Breakthrough filter system, and I bought the magnetic adapter rings, though I do not have the magnetic polarizers or other magnetic round filters. After using it for a while, I realized I much prefer the simple brass adapter rings, instead of the magnetic ones, because the brass adapters are flat, and I can stack them in my bag taking essentially no space at all. The magnetic rings have a conical profile so I can't stack them easily and they take too much space.
Unfortunately they discontinued the brass adapter rings, and now they only have aluminum rings, which are nowhere near as good as the brass ones.
Another problem I have with their system is that the screws on the X100 holder (the one that actually holds the square filters) come loose all the time. They are ultra tiny screws and they don't use phillips heads, so they are a total pain in the ass to screw back.
Oh, and yes, in case it's not clear, I use my own threaded polarizers that I attach to the lens, and screw the brass adapter rings to the polarizers. There's no vignetting or anything, as the brass rings are totally flat, so they don't push the filter holder and the square filters away from the lens. In fact, screw polarizer with flat rings might put the filters closer to the lens than the (conical) magnetic holder, though I have not measured exactly.
In any case, I am extremely happy with the quality of the glass itself. And the holder feels very nice, nicer than the expected competitors (though I haven't tried them all), but that screw keeps getting lost.