IMHO, the .357 SIG was a solution to a problem that did not exist. That being said, I did at one time own several guns chambered in the caliber and I think it was a noble cause to design the cartridge.
Pros:
If you're in the Sanow/Marshall camp, you get ballistics equivalent to the venerable .357 magnum 125gr. JHP which those authors document as the *best* ever one shot stop round in history. This has to be taken with a grain of salt though. At the time of their treatise on stopping power, the .357 125 was also probably the most popular round so it might have had some sort of statistical advantage over the others looked at due to more actual shooting data being available. Additionally, one shot stops were much more important with 6 shot revolvers.
Bottleneck case design. This has some theoretical advantage from a feeding reliability standpoint.
Cons: COST
In summary, I do like the idea of it and even owned it myself for a while however, it never really caught on and hence became just too expensive to shoot.
I guess if I had lots of extra disposable income and didn't care, I would probably use it. If I was only buying a single gun and wanted the best possible caliber (assuming you believe some of the studies out there) then I might still choose it. But, I don't think I would ever buy a .357 SIG barrel if I was already well armed with a .40 S&W.