I owned a ZI before I got a leica. My biggest complaint after switching, was that the viewfinder of the leica just looked so...meh.. in comparison. The ZI viewfinder is HUGE and its great for working with 28mm, etc.
I had the camera for a while and I ended up having it replaced by Zeiss due to the leatherette failing as well as an issue with the strap lugs wearing down.
If I shot more 35mm film i'd definitely consider getting a ZM.
I had a leica m7 shortly thereafter and it had tons of electrical problems, and after that I just went straight for the M6TTL.
Maybe if I ever have an extra 900 dollars burning a hole in my pocket Ill get another ZI. It was a nice camera for sure.