IMO, Understanding Big O helps you avoid doing stupid stuff like O(n!) where the problem can grow faster than your ability to purchase hardware to keep up. But

IMO, Understanding Big O helps you avoid doing stupid stuff like O(n!) where the problem can grow faster than your ability to purchase hardware to keep up. But you probably know when you are writing code like this intuitively. If you have very performance critical code you probably have some sort of range of inputs in mind. In which case it's more practical to just do benchmarking and statistical based evaluation.