It's a cartridge caught in the middle and released with bad timing. It's really a good round, but not a huge step up from anything already available when it was released.
It does seem to create some very good stretch or temporary wound cavities, and it seems to impart energy in target very well. I've seen a few tests now comparing the +P+ 9mm loads to the 357sig and while velocities are close, wound cavity and transfer of energy seem to be on the side of the 357sig.
I have had 2 pistols in 357sig, and I really do like the round, but tracking down ammo reliably and locally was a pain for some time. Now I can get it locally for $2-$5 less per box than 45acp, or just slightly more than 40S&W. That was the only reason I got rid of mine.