It's a neat cartridge, one of the most interesting new rounds of the last two decades. Bottleneck cartridges usually don't catch on with the American public, but this one seems to do just fine.
Pros: Very flat trajectory, high velocity and muzzle energy, excellent penetration, and duplicates the respected .357 Magnum out of a more compact autoloader cartridge. Many .40 *&* pistols can be readily adapted to the .357SIG just by swapping barrels, giving the .357SIG a huge potential platform base.
Cons: Expensive (still), very high slide velocities, and the drawback of shooting a 9mm-sized bullet out of .40-sized cases, thereby reducing capacity without going up in bore size. Also, the variety in .357SIG factory loads could be a little better.
I've owned two pistols in that caliber, a Glock 32 and a SIGPro SP2340. Both id just fine and were a lot of fun to shoot, but they both ended up on the trade-in altar somewhere down the line. I'd own another one, and probably will, if I ever get a good deal on one.