Over the ~16 years I spent working in bike shops, I think I may have seen one 'defective' tube. I have seen many cheap tubes fail due to rough handling during installation or inflation, and many fail due to not properly locating the cause of the puncture.
I have more recently noticed, however, that some of the cheap tubes branded by their distributors (MEC tubes in Canada, for instance) are of low enough quality that makes all other causes of punctures more likely.
As others have said, the name brand tubes like Conti and Michelin are generally very good. I have had good luck with Specialized tubes, ironically, because I have seen many defective Specialized tires.