Those Contax G lenses have such lovely rendering. I just never could put the investment into them because I wasn't sure I would like the adapted manual focus capabilities that are required to make them work. But, I liked them so much that I bought two Contax lenses in the C/Y mount. They are very nice lenses in their own right, but very large on our cameras. I remember when I fist started looking at Contax G lenses; their prices were half what they go for now. Perhaps I should have pulled the trigger.
I really want one of these Industar lenses, but not sure if they all require customization work to make them effective on the Sony. @Alowisney your pictures really do demonstrate great quality with the Sony, excellent work!
Mine had to be CLA'd (cleaned, lubricated, adjusted) because it felt like it was full of sludge and sand. During that process of repair, it was also checked to make sure it focused properly. I just put a Rainbow Imaging M-39 adapter on it and it works great on the NEX. I would suggest buying from a dealer like Fedka so if anything goes wrong with it you can get it replaced. I had a great experience with Fedka and have never read anything bad about them. You can also check with Oleg with OK Photocameras. Oleg is outside of the US though.
Edit to add: Don't forget about the EXCELLENT Contax copy lenses that came out of the USSR. The Helios lenses are amazing, I keep meaning to get one (or two)!