Carried the 649 for several years. Replaced it with a 642-1, which is the newest aluminum Centennial with stainless cylinder & barrel, rated for use of +P ammunition.
To ease manufacturing, S&W has lengthened the frame cylinder window in the newest little guns, even those only chambered for .38 spl, so they can install the longer .357 cylinders in those models rated for magnum. On the .38-only guns they just install the longer cylinders bored for the shorter .38 spl rounds.
I paid $315 for my 649 several years ago. That was back when S&W revolver QC wasn't what it had used to be, and my gun required immediate armorer attention. The newest ones, though, have been working great right out of the box, as they should, around our neck of the woods ...
If you're considering a used model 38 or 638, they're great little pocket guns, but I'd have it checked out by an armorer or gunsmith, just to make sure it functioned properly.
For just a little more money, though, check out one of the newer 642-1. The enclosed hammer means you won't have to clean out lint in the hammer space like with the Bodyguard series