I have a Model 60-15 .357 mag with the internal lock. When I first purchased the gun, I oafishly knocked it (unloaded!) off a counter onto a hardwood floor. When I picked it up, the gun was totally locked up (although the "Locked" indicator was not raised up). I was pretty distressed, until I remembered the internal lock. After I dug out the "keys" that came with the gun, I locked the lock fully and then unlocked it, and everything returned to normal.
Now, I'm not necessarily down on S&W because of this one problem I had. After all, I was the numbskull who knocked it 3" to the floor - something that is probably a little out of the gun's usual specs for performance.
On the other hand, I can see there being a situation in which one might drop one's carry gun (say, in a struggle) and then need it to function in a hurry. So, I'm not thrilled about what happened, or indeed about the lock even being there.
The upshot is that I've stuck one of those stupid "keys" on my keyring, just in case. That and I never carry the 60-15 by itself - always with another J-frame as back-up.
(Other than that, the 60-15 is a heck of a nice gun!)