The first successful polymer gun, the Glock, has to be the most thoroughly tested gun in history. From the early days when the anti gunners were saying it would pass through airport security without being detected, the 100,000 round marathons, and all the FBI tests before they adopted them, Glocks keep passing all the tests thrown at them.
The XDs, MPs, and all the others might wind up being just as good, but it will be years before we really know.
For standard configuration automatics (non-striker) Sigs are pretty darn good, but I'll stick with the well proven Glocks when my life is on the line.