357 Sig is in the same category as 10mm to me, if loaded to to what I call "true" specs (aka hot spec ammo) it has 357 magnum ballistics with 125 grain bullets however (just like 10mm) most commercial loadings are watered down which negates most of the advantages of the round. Why? Probably because its louder and has more recoil when loaded to "true" specs. Without going off on a rant, that is the exact opposite of what is popular and trendy right now.
I don't have a 357 Sig chambered gun yet, but YET is the key word there. I should just buy a 357 Sig Glock barrel and start playing around with it, but I just bought my first 10mm and other things in life have taken a priority. As far as I'm aware, it has the most energy of any handgun round that can fit into a 9mm/40 cal size frame which to me is very appealing. Obviously other people disagree but that's why there are so many different calibers out there to choose from. If it interests you, buy it and try it. If you enjoy it keep it, if not sell it and move on to something else you enjoy more.
If you get into reloading for it, you'll be able to realize the full potential of 10mm, as long as you use the right powders.
AA#9 is what really gets the most out of it for me, and I have no trouble getting 700+ FPE using it and either 155 or 165 grain bullets.
It's nice to have a "maximum effort" defensive platform should push ever come to shove.
Reloading is "in the works" but it is going to be a very long and slow process to get there due to lack of free time where I can actually devote to learning all of the inns and outs. Once I do get up and running with reloading I can see it being easier to justify playing with rounds like 10mm, 357 Sig, 38 Super, 458 SOCOM, maybe even 40 Super, 400 Corbon, 45 Super, etc but I know I need to master the basics first. 45acp is one of my favorites and I've heard is fairly easy to learn to reload so thats where I plan on starting.