I personally favour only commenting on the unusual. I also think that if you are doing TDD then the tests should do a good job of documenting the code. I can't count the number of times someone has asked me to explain some code I have written where my first response is "okay, let's look at the tests".

All that being said, it really depends on the scenario. My second last project was a shrink wrap product development that ran a million dollar plus accompanying piece of hardware. This project quite rightly required us to write thorough and consistent comments and docs. Expensive product, complex code, high risk = thorough docs. I am currently working for a small rapidly growing and changing business, where the software is mostly internal. Low risk, quick development required, constant change, low complexity = waste of time and money creating and maintaining good docs