Yawn.... as you ask for opinions on the .357 SIG as a cartridge to walk around with, I'll try to avoid mention of other rounds. Which isn't easy, as demonstrated by earlier responders. All cartridges have parents and ancestors; and for performing tasks most have competition .
Soon after SIG/Sauer of America released the necked-down .40 S&W, a couple of LEOs asked for my take. Having no experience with the round, I shrugged it off. Immediately upon having a SIG P229 in my hands and shooting it, I commented an appreciation for its accuracy and low recoil. Trying at the time a spectrum of factory loads, only one came up short. Hornady 147 JHP produced rather miserable accuracy. I suspect the long 147 grain bullet doesn't like the very short neck of .357 SIG case.
Federal 125 JHP produced superb accuracy, to the tune of 5x5 into 6-inches @ 100 yards from the stock 3.8" barrel of the P229 with Trijicon sights. Other rounds groups close to the Federal, including Hornady 125 JHP. No surprise to me, FMJ bullets were not quite as tight. Cor-Bon 115 and 125 grain JHP produced higher velocity, with diminished accuracy. Nevertheless, it was possible to keep all shots on a man silhouette @ 100 yards.
Took the P229 and Federal .357 SIG 125 JHP to an IHMSA silhouette championship in New Hampshire, having never shot a chicken, pig, turkey, or ram with the little pistol. Shooting Creedmoor, the stinking 3.8-inch barrel and fixed sights toppled 7x10 turkeys at 165 yards! Prior to that, I hadn't fired a round at over 100 yards. 5x10 or 6x10 rams bit the dust @ 220 yards.
The .357 SIG seems perfectly scaled to a .40 S&W-length frame, slide, and magazine. A SIG .40 S&W barrel drops right in. Same gun, same recoil spring, same magazine, same perfect reliability. (.40 JHP groups of five shots average around 9" @ 100 yards, with more drop.) I consider the P229 .357/.40 and 2-in-one gun.
The .357 SIG is less interesting from a sub-compact Glock 33 or 27 where, in comparison with the .40 S&W, recoil of the .357 is rather nasty. The P229 tames the .357 SIG, and does so with a far less blast than a .357 Mag.
David Bradshaw