The concept is not rubbish, the interviewing technique is. I can think of any number of people that can rattle off what O(n) means, yet inevitably write O(N^3)

The concept is not rubbish, the interviewing technique is. I can think of any number of people that can rattle off what O(n) means, yet inevitably write O(N^3) code given half a chance. And vice versa. And another group that forget the constants that big oh notation throws away very often really do matter. Talk about efficiency, sure, why not? Throw a piece of code at somebody with convoluted terminating conditions on a loop and ask for the big oh, under pressure, while being worked? Rubbish.