when you say "Leica Barnack" what do you mean, any of the Screw Mount cameras? Absolutely they're good users -- the separate range/view finders are not quite as accurate as the M-series frames, which are parallax compensated. A compensation is that the Barnack Leicas are a lot smaller and lighter than the M-series, and the viewfinder is clearer because it doesn't have any semi-silvered mirrors in it to darken things for the rangefinder patch.
I've got several and haven't had one yet where the rangefinder had de-silvered so you couldn't see it -- this includes a post-war IIIC which is 60-plus years old at this stage. Shoot for a IIIF and have it serviced, you will have a very reliable little camera.