I'm a fan of the .357SIG personally.
From my experience, the .357 is more accurate than most other calibers I've shot. My P226 has turned in some amazing groups. I think, due to the bottle necked design, it also feeds smoother than other rounds. When I put my .40 barrel in the .226, I can manually hand cycle the slide, and if I do it slowly enough, I can get the slide to hang-up on a .40 A-Zoom snap cap. That NEVER happens with the .357 barrel. The action just seems to cycle smoother with the bottle necked case.
Also, while the best +P+ ammo can approximate .357SIG performance, keep in mind, the average 9mm is build around +P ammo, NOT +P+ ammo which isn't SAMMI spec'd. Therefore, the average .357 will have a heavier slide and springs than most 9mm pistols. As a general rule, I just don't run +P+ ammo in any of my 9mms. Further, when you're pushing 9mm to +P+ pressure levels, the accuracy tends to diminish.
.357 can be had at reasonable cost when bought in bulk from a number of on-line vendors. I think it's ballistics are impressive and I also like the flexibility to switch between .357 and .40 with ease. If you have a P22x series pistol in .357, it can easily be switched to one of three different calibers with the addition of a Bar-Sto 9mm conversion barrel and the factory .40 barrel. Recoil wise, I think the .357 is actually more mild than most .40 loads I've shot. But that's a subjective call I imagine.
While the .357SIG may not be for everyone, I really like it. :?