I have a P220ST, and had the extractor problem. My understanding is that it was due to a bad run of extractors from a supplier, and affected P220ST's manufactured a little more than a year ago. Only the extractor is the issue, nothing major in the slide or frame.
From what I've read (and my experience), IF you have the bad extractor you'll know within about 500 rounds. If you do have a bad one, SIG will of course repair it under warranty and even pick up the shipping. I took advantage of that free shipping to have their custom shop do a trigger job and some other work while it was there.
In short, I wouldn't let fear of getting one with a bad extractor stop you from buying one if you're interested in them. If you even get one with the problem, SIG will definitely take care of you with their fantastic customer service.