I acquired a Leicameter MR-4 for my M4-2. Took it for the first walk today.
It is such a pleasure to use! I almost prefer using it over the in-viewfinder metering of the M6 and M9. I like being able to look down at the top of the camera and seeing the meter reading, all the settings, etc, in one glance.
The whole camera is a pleasure to use. Just the right size, just the right viewfinder, not overly heavy yet substantial enough to feel good in the hand.