I have HK x1.25 magnifier, used to have it attached to my M6 when shooting with my 90mm Elmarit. Indeed makes 90mm frame bigger, so is the RF patch coverage. Helps in focusing. However, the downside of it is mainly for eyeglasses wearers. The magnifier's edges are hard metal one and I would image would easily scratch the glasses. Can be fixed by some kind of rubber o-ring or something like that.
I found 50mm frame thruogh the magnfier to be approx. like 35mm without the device, perhaps even slightly bigger, hence a bit harder to see, though again, this is mostly relevant for those who wear glasses.
Another thing worth to mention - the screw throw of the magnifier is really short, take approx. 1/4 revolution to put it into M6 eyepiece. Admittedly, doesn't inspire confidence at the beginning, but in actual usage I found it sitting qute snug and secure. Had never loosened so far...
Having said that, I found ultimate solution for confortable long lens usage (50 and upwards) - an M3. Since getting a user M3, the magnifier mostly sleeps in its pouch humbly, has rarely taken it out ...