Russell:
I would say that the #1 reason that the .357 Sig has cought on is simply because it can be launched out of a 9mm frame.
The 9x21, .38 super and others have the same ballistics, and were a good attempt, but they require a large frame handgun to fit the long cartridge.
The .357 accomplishes all this out of a small gun, that is easier to carry, and fits the hands of more people. As you can see, I am quite enamoured with it.
I think this round is a shoe-in for law enforcement due to it's deep penetration and excellent barrier penetration, with enormous kinetic energy, low recoil and all out of a small platform that will fit many LEO's hands. All these characteristics make it a very good law enforcement round. I suspect we will see a lot of LEO's going to it. Unfortunatly, the inferior .40 caliber beat the .357 Sig to the punch, and so many people have gone to the .40 recently, that they are not ready to switch again. So, it may take a few years before we see the majority of LEA's switching to the .357 Sig, after their new .40's have worn out.
So far, the street results for this round have been stellar. Many street shootings so far show that it drops felons on the spot. When you are dumping 600 foot pounds into the bad guy instantly, coupled with 15.5 inches of penetration, then you can see how that might drop a person on the spot, with even better ballistics than the old .357 Magnums.