There's a large selection bias reading these forums - only people who have problems will post about them. The majority of people with perfectly fine machine will never post anything for you to read.
Likely, if there was a significant defect rate, the mainstream media would have picked up on it.
Are some defective? Sure, Apple sells millions... at least one has to go bad. But it's far more likely if you bought one you'd get one that meets all quality specifications.