I have had the 35f1.7 since it came out. It is looking quite worn by now, I have to admit, but I have never had the front group unscrew! I did read about the problem on RFf and other forums. I suspect that part of the problem lies in the unscrewing of the hood and filters. Mine is an early, black version and the anodizing is wearing quickly! My only beef with the 35f1.7 is the ergonomics. The tapered front, towards the aperture ring made me miss shots. beacuse when I though I had the aperture ring, I was trying to turn the barrell!
Optically it is a very good lens. It has less of the Aspheric harshness of the Leica 35's and is fare less flare prone. It is out of production now, so I suspect that we will see a new 35 medium speed lens in not too distant future from VC. This will probably be a M-mount though.
Roger, interesting with your 50f2.5 having the focus tab coming undone! I have several of these lenses and thats a first I have heard about it. This is a bit of an unsung hero of a lens. Very good and the fact that it looks and feels like a 35f2 Summicron can be a bit disconcerting.
As for unscrewed fronts, I had more problem with that with my 35f1.4 Summilux and a couple of 50 Summiluxes. Nothing a little Loctite could not fix though!
One of my S-A 21/3,4 had a tendency to come apart too. The screws holding the front group kept vibrating loose when I was doing a lot of "bush" flying in the interior of British Columbia and Ontario (The De Havilland Beaver was not known as a vibration free aircraft!). Once, I fixed the lens in the north of Quebec by borrowing nail polish from the receptionist at a mining operation. Once back, I took the lens to Leica to redo and tighten it. A couple of days later I get this aggravated phone call from Leica Service " What the hell did you use to hold in the screws with". I said "Black Nail polish" Leicas response " Thanks, we have destroyed about $200 of premium screw drivers on it and it still wont came apart"