i have a p6 with the 80mm lens. it is a very good camera capable of producing great photographs. i have only run into problems once when i didnt load the film properly (tautly enough) which resulted in some pretty horrendous frame overlap. have a look at http://www.pentaconsix.com/ - the website has lots of good info on the p6, especially film loading.
Just got one with the 80mm, 120mm and 50mm lenses and haven't done much with it yet. Interesting beast, with a darkish viewfinder and the shutter is reasonably quiet for a camera of this sort. Like all Pentacon products I would imagine the reliability could be suspect, so I won't flog it too hard!
The first roll through would indicate that the lenses are all pretty damn good optically, although there is some looseness in the aperture rings (the 120mm is quite bad but useable). Mine is the P6 TTL BTW.
The bad reputation of P6es comes form ebay - here problematic exemplars are concentrated. If you buy a well working one you should not have any problems, over many years. I photograph with my six more than ten years an did not have any problems. The only thing you should consider: If you advance film press the release button during the first half turn of the lever. This avoids overlapping.
TRA is very friendly and knowledgeable about the P6 system, a great resource indeed.
I am a big fan of this system:
If you buy from a reputable seller, camera should not give you any trouble, as long as you follow the instructions carefully. Making sure film is wound tightly, and not letting the advance lever return with full force are the two main considerations when using the camera.
Final units can be identified by the plain release button (all others have engraved circles) and all-metal tripod mount (others have leatherette coverings), so that should ensure you get one of the last cameras. Depending on the sources, there were around 207k or 205k cameras made.
Lenses are all excellent, my favorites are 50 and 180 but all of them are great glasses. The system has plenty of accessories, fresnel screens can be had for about 20 USD, and there are still repairmans out there willing to have a look at our cameras.