I have shot it a bit and couldn't find any discernible reason to use it other than because it is different. Sometimes, mind you, that's a good enough reason. But ammo availability, cost, and reloading component availability (you'd almost never get free 357 Sig brass!) make the cartridge a bit of a tough sell. In action pistol competition I can't see any real advantage over .40 S&W, especially for the handloader, where you can load to minor or major power factors.
I'm not saying it's a bad cartridge (on the contrary, it's actually pretty awesome) but for the applications I have for handguns, I can't really justify it. If I had more money to throw around, though, I'd definitely be eyeballing a P229.