Big-O Notation

Big-O Notation is a kind of a strange foreign language. Interviewers use it in order to show that they are smarter than you, the development managers mention it to demonstrate own IQ to the programmers. :)
In case you have to know what is this Big-O notation read these articles in the following order:
1. A Beginners’ Guide to Big O Notation.
2. An Intro to Big Oh Notation with Java.
3. Complexity and Big-O Notation
4. An informal introduction to O(N) notation
5. Algorithmic Efficiency -- Analyzing Algorithms
6. Algorithmic Efficiency -- Various Orders and Examples
From my point of view it's a bureaucracy that comes to the real programming. Yes, it says something about the algorithm performance. But it can be a real mistake to write a fundamental algorithms yourself in complex commercial projects. It's important to see in the source code the bottle necks, but if I see a nested loop, I do know that it is better to "open" it and the fact that now I know that is O(N^2) does not help me at all.