I have the 640 .357 Magnum but have been able to handle examples of the others (9mm, .38 Spl) and they all seem to be good little pistols. My opinion based upon my experience is that they are a tad heavy for pocket carry for me, but they will fit in the pocket easily. I use an Uncle Mikes pocket holster and it works well, plus is inexpensive.
Try C R Sam's theory and walk in with cash and see what the dealer will do. AFTER you run the standard revolver check that is. If it checks out oaky and the dealer does not seem to want to budge, put it down, kinda look at it, and say, "It's kinda beat up lookin"." and see if he will negotiate.
Oh, I prefer other grips which fill the area in front of the handle more. Hogue makes a good grip, I forget which name. Eagle (Secret Service?) grips are also good. The standard S&W j-frame small grips just don't give me enough of a firm grip.
Just decide before hand what niche this will fit in your handgun battery and just how much it's worth to you.