I've got a Steyr M357 and a Kel Tec P-357. The Steyr is mild to shoot with the low bore axis, but the Kel Tec is a hand cannon.
The caliber is flat shooting, accurate, and has excellent penetration.
With that said, I still prefer the .40 S&W over the .357 Sig -- simply because I want a larger and heavier bullet, as well as the availability and cost of the ammunition.
In terms of comparison, the PowR'ball load is one of my favorite self defense loads -- along with the DPX.
The PB produces the following:
9mm+P 1475 fps 483 ft/lbs.
.40 S&W 1275 fps 487 ft/lbs.
.357 Sig 1600 fps 568 ft/lbs.
So, you can see the added energy produced by the .357 Sig.
The following link has a great deal of information regarding the cartridge -- and what agencies have adopted the loading in duty weapons.
http://www.handguninfo.com/Archive/www.Pete-357.com/