I recently bought a Kiev 4am and Kiev 2a. Thus far I have Helios 103, Jupiter 8 and Jupiter 12 lenses to use with them.
When using the Jupiter 12 (35mm) I use the KMZ 35mm aux viewfinder, and it is fantastic. It seems completely natural to compose/frame using the aux, then use the rangefinder to focus.
I find myself wishing for a 50mm aux (or UF), just for the clearer view--especially in dim light. Pretty sure there will also be an 85mm aux viewfinder in my future, along with a Jupiter 9.
These things all work very well together. Nothing is really "wrong" with the combination viewfinder/rangefinder (for 50mm lenses), but an aux VF is a serious enhancement to the system.