I have 3 with locks. So far no problems due to the locks. The quality of the newer guns is not up to par with the older guns. With a little polishing and some new springs they can be made very smooth with lighter trigger pulls. That being said, I recently bought a new no lock 942, I also have an older 642 with the lock. I had to install a spring kit in the no lock gun to lighten the trigger pull. The no lock is now as smooth and light as the one with the lock. Both are totally reliable so far, I've shot the no lock more than the other one but both are as alike as two peas in a pod.
The no lock is my carry gun, the other is a car gun.
The gun with a lock that I've shot the most is a 625 JM. I added springs when I got the gun, it's a range gun, shot IDPA with it, it's put a lot of rounds down range, I guess it's my favorite to shoot.
The 625 and the 642 with the locks do not have the BUMP near the end of the pull, 686 and the 642 no lock do have it, I HATE it. Agree with Willie. It's about gone in the 642, I'll never shoot the 686 enough to wear it in.