The 640 features a steel frame that ups the weight on the gun by about half a pound. That's too heavy for most people to conceal in a pocket, but much more pleasant to shoot at the range. It's a great training aid, sparing both your hands and your CCW gun from the wear and tear of recoil.
Sights and Trigger
The "Pro" model comes with a couple bells and whistles over a normal 640, including front and rear tritium sights and a cylinder cut for moon clips. The dovetail sights are pretty much the best available for a J-frame; they're tough, low-profile, and visible in all light conditions. The moon clip-compatible cylinder is nice to have, but not significant for most people.
One thing the "Pro" model does NOT have is any trigger or action work done on it. In fact, this particularly 640 Pro had the worst trigger I've ever experienced on a J-frame, a very heavy and creepy double-action. I had a local gunsmith work it over, and also added a Pachmayr Compac grip (too big for pocket carry, but perfect for a steel-framed revolver).
Range Report
Theoretically, you can shoot .357s in the 640 Pro, but the concussion and recoil from this 2-1/4" barreled snub is so severe that it's really impractical to use .357 ammo, including for self-defense, unless you have no other choice. I carry and shoot .38s and .38 +Ps out of this gun, which I think will serve most people well.
At 10 yards, the 640 Pro shot decent groups with old-school 158 grain lead round nose:
Things get spicier with the Federal HST Micro .38 round, which has a flat ogive hollowpoint giving it a profile similar to a wadcutter. This is one of the newest, most advanced .38 Special self-defense rounds on the market, and it shot reasonably well once I got used to the recoil.
Conclusion
In my mind, an all-steel 640 Pro is a very specialized gun. If I'm going to carry a 22 ounce piece on the hip, it's going to be a modern slim polymer 9mm, which holds more than twice the ammo, is more accurate and powerful shot-for-shot, and easier to reload and maintain. On the other hand, if I need to throw down a couple boxes of .38 Special every month to keep my snubby revolver skills in shape, the 640 Pro is one of the only choices on the market.