I haven't shot a Model 639, but I own a M3904, a similar and later version.
The M639 is one of the so-called 2nd-generation models from S&W's "Traditional Double Action" lineup. It's basically a development of the earlier Model 39, although it has a stainless steel frame instead of the alloy frame used on most M39s.
The 2nd-gen pistols incorporate a positive trigger-actuated firing pin block; unlike the 1st-gen pistols that lack this feature, 2nd-gen guns cannot discharge if dropped sharply on the muzzle with the decocker/safety lever disengaged, so they are much safer to carry in DA/safety-off mode. Most 2nd-gen guns also have a RH-side ambidextrous decocker/safety lever, although this was optional for a few years, so not every gun has it. Unfortunately, the RH-side lever on earlier 2nd-gen guns is retained by an oversize and hideous Philips-head screw, which is why you'll find suspiciously few pictures of the RH side of 2nd-gen TDA pistols on the 'Net.
The lever retention system was later changed to a tab retained by a spring-loaded pin, which was used on all 3rd-gen pistols with the RH-side lever.
The 2nd-gen guns retain several 1st-gen features that were dropped on the later 3rd-gen pistols: separate grip panels retained by screws, a separate M1911-style mainspring retainer in the backstrap, and a separate barrel bushing. However, the pistol is not field-stripped exactly like a M1911, although several parts of the procedure are nominally similar.
These pistols are NOT the easiest to field-strip, although they're not terribly difficult; there's no "secret handshake" like with, say, a Ruger Mk series.
Problem #1 is that the slide has to be partially held back against recoil spring pressure to line up the takedown pin (slide stop) with the takedown notch, both during disassembly and reassembly; this is much easier with the hammer cocked, but it's never truly
easy. Problem #2 is that 3 fiddly little levers at the rear of the frame have to be held down to reinstall the slide, and a couple of them are notoriously fickle.
Almost all standard-production TDAs have a mag disconnect. There is a way to disable it, but IIRC it gets quite mechanically involved, and most TDA shooters don't bother with it since the mechanism does not interfere with the trigger pull like with other common auto pistol designs. Basically, if mag disconnects truly and deeply bother you, I would recommend looking at a different kind of pistol.
The 2nd-gen 39-series guns will take any mag from any full-size 39-series gun, but NOT the 39xx-series compacts. Used 8rd mags from the M39 are quite easy to find and inexpensive. 9rd 909/952 factory mags will also work, although prices start at ~$33/ea and go up from there, and used ones are few and far between.
S&W TDA pistols shoot very well and are typically very accurate, although the loooong and heavy DA pull takes practice. The SA trigger will have some crunchy slack due to the firing pin block disengagement, but is usually very crisp once you're through the first stage. Many shooters find that the trigger reset is the standard against which all other autos are measured.
It's VERY short, VERY crisp, and extremely easy to adjust to.
A popular TDA mod is to substitute a 16lb-18lb Woolf mainspring for the stock 20lb-22lb spring. This yields a lighter DA trigger pull, and the gun will usually stay close to 100% reliable using factory ammo with these springs. (The factory switched to an 18lb spring on later 3rd-gen guns; it's believed that the heavier springs were originally used for safety reasons and/or LE agency preferences.)
The 2nd-gen guns have some issues with sights. Issue #1 is that the front sight is integral with the slide and therefore cannot be easily changed; if you want something different that what comes on the gun, a gunsmith's attention will be required. Issue #2 is that the optional rear sight was a big, ugly, and not terribly CCW-friendly fully-adjustable unit with rounded metal wings. This sight is great at the range but not for carry, and AFAIK you CANNOT substitute any other type of sight because the dovetail in the slide is MUCH wider than the standard sight dovetail. IOW if you don't like it, too bad.
Most M639s were built with a rounded trigger guard like the M39. Readily available M39 holsters will fit, and many Colt Commander (or generic 4" M1911) holsters will also work. However, late in production, S&W switched to a squared-off trigger guard that generally requires a specific holster. Round trigger guard guns will fit in square trigger guard holsters, but usually not the reverse.
OK, what else do you want to know?
