I suppose you could say it's good if you like using Kievs but don't like the squinty viewfinders of the Contax-based models. The projected-frame viewfinder is a bit odd (framelines are slightly parallelogram-shaped) and not super-bright, but certainly very usable.
You're right, the lensmount is non-standard; it has both inner and outer bayonets, but no inner focusing
helix, which means that it can use any previous Kiev/Contax lens
except the normals; these used the built-in helical, which the Kiev 5 lacks. Instead you have to use a Kiev-5-specific normal lens which has a focusing helical built into it. This limits your choice of normal lenses somewhat, but the 50mm Jupiter and Helios lenses available in the Kiev 5 mount seem to be pretty good ones.
The other distinctive "improvement" of the Kiev 5 over the older models is its rapid-wind lever, but this feature is probably the most controversial. Some people say it's too easy to strip the advance gears if you wind this lever too quickly, and suggest instead that you always advance film in the same fashion as the older Kievs, by turning the shutter speed/advance knob. I usually did this, but I also did use the lever on my Kiev and simply took care to operate it gently. I'd have to say that the lever has such a long, heavy stroke that it doesn't really speed up film advance very much compared to using the knob.
Other problems I encountered with my Kiev were occasional blank frames (probably just because of its age, lack of maintenance, and/or sloppy Soviet construction) and the fact that the focusing ring of the lens would bind occasionally (caused by a spring-loaded ring inside it, which was necessary to couple to the RF mechanism; sometimes the spring would stick.)
On the whole I would describe it as "quirky" rather than "convenient," but if you already like FSU RFs, it's certainly a very usable one and is certainly distinctive! In fact, I would say that if you want an FSU RF with comparatively modern features, the Kiev 5 would be second place only to the DRUG, which is a screwmount camera and not easy to find.
I no longer own my Kiev 5 (sold it to another RFFer; hope it's enjoying a happy life) but still have some pictures of it at this link:
Kiev 5 pictures
If you click the small pictures to get to the larger view, there is some brief caption info as well. Most of this was written for the person who bought it, but might be interesting for others.
From that page, here's a picture of the lensmount, so you can see the bayonets and the arrangement of the back of the Helios lens:
