I'm the proud owner of a third generation S&W (1006!) with a decocker that doubles as a safety if left in the down position after decocking, and I really like it a lot, even though I never leave it down to act as a safety, since decocking puts the trigger into double action mode.
Others on this thread have voiced misgivings about how safe a decocker is, but I would point out that I invariably decock only after I ensure that the muzzle is pointed in a safe direction.
I've never had an issue that way, and it's considerably safer than trying to gingerly lower the hammer after squeezing the trigger.
If I were looking at getting one of the few DA/SA hammer autos with a choice between decocker or safety models (the Beretta PX4 Storm jumps immediately to mind), I'd get the decocker model and never look back.
It's an excellent system and certainly superior to trying to disengage a safety should the moment of truth ever arrive.
The 3rd gens were/are built like tanks and I've never read the decocker/safety failing. I have 4 of them, and all of them function great and look sexy doing it too. Out of them all the 5906 is my least favorite. I sold the first and had my regrets, so i got another. Its not as bad now that I changed the curved backstrap for the straight, but still not my favorite. If I see something I really want and think of trading I told my wife to take it from me and keep it for herself as she likes it more than me. I speak very highly of the 3rd gens.