My experience with a 15 stimp was, as mentioned earlier, at Chicago GC a few years ago. When I first stepped on the putting green, I could tell they were very fast. I tossed a ball down, picked out a hole about 25 feet away, and figured I had to hit it very soft. I tried my best, and it still went at least 15 feet by. That was Wednesday of Ryder Cup week, and my host said they were measured at 15 that Monday. I obviously have no first-hand knowledge on its authenticity, but I think my host is a very respected guy whom I have no reason to doubt.
It took a while, but I got used to the speed. And after a few holes, I quite honestly didn't think about it. I just got the feel for how hard to hit each putt, and it seemed to work out. After a bogey on #1, I parred 2-8, so it is doable on those greens with those speeds. I have never played Oakmont, so I can't say if their greens are sloped more or less than CGC. But on the day I played CGC, the green speeds were acceptable. You just can't let your guard down on any putt, especially a down hill putt. And Sean...when I think of it, I realize I got up-and-down for pars from short grass, not rough around the green (with the exception of back-right behind the 8th). I seemed to miss short if I missed, and therefore didn't have to contend with much greenside rough. Probably one of the reasons why I was okay with the speeds.