I have two SIG P229s chambered in .40 S&W and a 357 SIG barrel that I sometimes use in one or the other. The cartridge is fun to shoot and is certainly a potent self-defense cartridge but the question is whether it offers any real advantage over the alternatives.
There are some things about the 357 SIG round that I know are true.
1. It is loud (in comparison with 9 mm, .40 S&W, .45 ACP).
2. It is more expensive. For those who don't hand load, commercial FMJ practice ammo is going to cost more than .40 S&W and a lot more than 9 mm Luger. If you can buy on-line and especially in bulk, you might be able to get FMJ 357 SIG ammo for about the same price as .45 ACP. If you have to buy locally, it is almost certainly going to be the most expensive of the four calibers mentioned, if you can find it at all. JHP self-defense 357 SIG can often be found at prices comparable to the other calibers, but the selection is more limited.
3. It has more muzzle flash. The muzzle flash might be a significant drawback if you have to shoot in dim lighting and the louder report might be if you have to shoot indoors without hearing protection.
Balanced against these drawbacks are some alleged advantages.
1. The bottleneck cartridge is less likely to cause feed issues. I really don't see this as much of a deal. None of my .40 S&W pistols has ever had any feed issues so I can't improve on 100%.
2. The cartridge might have better barrier penetration than .40 S&W. Allegedly, this is a big part of the reason that it was used by Federal Air Marshals (who might need to shoot through airplane seats and partitions) and Secret Service (who might need to shoot through vehicle doors and windows). In most civilian SD scenarios, better barrier penetration might not be an advantage.
3. Being a higher velocity cartridge, it will shoot flatter at distance. This will only become significant at ranges over 25 yards. Again, probably won't be an advantage in most civilian SD scenarios.
4. It delivers more kinetic energy. Here is where the fur starts to fly. Those individuals who believe that kinetic energy deposit is the Holy Grail of handgun terminal ballistics embrace 357 SIG even through its performance in ballistic gel with regards to penetration and expansion is no better than that of some 9 mm loads and inferior (wrt expanded diameter) to .40 S&W loads. Commercially available 357 SIG JHP loads have about the same momentum as .40 S&W loads. There are a lot of 357 SIG fans who claim that damage to small animals they have shot with 357 SIG is more extensive than with 9 mm or .40 S&W. There are also a lot of anecdotal reports of animals going down more quickly with 357 SIG. I don't shoot animals, so I can't comment.
5. I have heard some comment that 357 SIG is easier to shoot than .40 S&W. I don't find this to be the case. Perhaps that is because I have shot a lot more .40 S&W than 357 SIG. I would say the recoil characteristics are similar, but different. I find 357 SIG tends to have a sharper jab-like push straight back into the hand whereas .40 S&W has a little more muzzle rise. Either definitely has more recoil than 9 mm Luger in a similar size and weight pistol.
Here are some ballistic gel tests of 357 SIG JHP loads which can be compared to 9 mm Luger or .40 S&W tests using the same protocol:
https://www.luckygunner.com/labs/357-sig-gel-test/
Paul Harrell is also good for entertainment, if nothing else:
http://www.youtube.com/watch?v=lrcSO5wErJw
I am a fan of .40 S&W. I have been looking for one or more definite advantages of 357 SIG over .40 S&W to compensate for what I see as its definite disadvantages (flash, noise, cost) and I have yet to find any convincing ones for civilian SD use. I think part of the reason we haven't seen more use of 357 SIG is that others have basically come to the same conclusion.