A good friend of mine carries the Glock in .357 Sig. Nice shooter, and it does a very good imitation of a .357 Magnum on steel plates. I sometimes shoot at the same range that the local Air Marshalls use, and the ground is covered with .357 Sig nickel-plated brass the next day, as well as a couple dropped loaded Gold Dots, so that's what they are using.
Texas DPS (including the Texas Rangers) moved to the .357 Sig to regain the penetration they used to have with the .357 Magnum when shooting at cars. They had gone with the .45 ACP when they moved to automatics.
My personal opinion is that it is a little too much gun for average self-defense. It's hard to imagine a self-defense scenario that has one needing to shoot a threat that is behind serious cover. This is much more likely a possibility for police, however. But as long as you are aware of this penetration and can plan accordingly, I won't tell you not to use it. After all, there just may be a time when it could come in handy. And yes, I feel the same way about the .357 Magnum. When not carrying my .45, my .357 snubbie is loaded with .38 +P loads except for the last round of magnum hard-cast.
Flame on!