Well, looking at the current stats, I believe Sony have said that 0.4% of consoles have/may have issues. They have stated today that they sold one million consoles on launch day.
0.4% of 1,000,000 is 4,000. It’s likely that a portion of those 4,000 people would take to the interenet to complain, like they have, giving one star reviews and what not, but that most will simply contact Sony/Amazon/Etc to get it replaced. So, as has been stated in other comments, at the moment it is likely that less than 1% of consoles are having issues.