So the 640 has some unique features going for it in that it is an all stainless snub nose .357.
While I might think it neat for collecting, here is why I wouldn't carry it: .357 out of a 2" barrel really doesn't perform all that amazing, you need more barrel to get a full powder burn and take advantage of the velocity. This with the added blast of a .357, where it's going to look/sound like you touched off a 1/4 stick of dynamite in your hand...So then perhaps a 38 Special +P? Well, there are plenty of lightweight 38s that will deliver the same performance as the 640, but save you almost 8oz when carrying it.
I'm not saying the 640 is useless. It certainly is easier to shoot than an alloy .38. However, if I'm carrying a small, lightweight gun for comfort I want it to feel like a small, lightweight gun.
I've carried both a 637 in a IWB, and a 4" 686 OWB when I lived in a city, so I bracketed the 640.