This is a very solid camera, and a large system of lenses and accessories is build around it. I have been using the camera for a while now, and there are only two disadvantages that I can think of:
- a specific film loading routine that needs to be followed (described in detail here),
- significant camera shake due to heavy mirror -- because of that, I never go below 1/60 without a tripod with this camera.
The first thing above is actually not an issue at all; all you have to do is learn the right loading procedure. The procedure needs to be followed simply because Pentacon Six places frames very close to one another, and if you do not follow the procedure, frames will overlap. The good thing about this tight spacing between frames is that you can usually fit 13th frame on a 12-frame film.
The second thing above can be fixed by modifying the camera to add the mirror lockup feature (a German shop does it, google it).
Other than the above, you should enjoy this excellent camera. Even though built in former Soviet bloc, the camera is up to Western standards in build quality, precision and handling.
It has very good lenses too. In particular, apart from the "kit" Biometar 2.8/80, try to find and buy the Sonnar 2.8/180. Some consider it one of the best portrait lenses ever created. Here is an example:
Pentacon Six TL, Sonnar 2.8/180, Portra 400VC