Well, I spent a lot of time trying to resolve the inconsistencies between my temps after reading the guide linked above. Then I recently read the quote above, and, as I say, started to feel much better about things.

Based on everything I have read so far, my take is this:

The temps reported by all these software tools are useful as an indication only, but we have to accept that they are, in all probability, not accurate, as the sensors themselves have probably not been calibrated to provide an accurate measurement. However, poor calibration shouldn't (in theory) make the readings totally unreliable (there must be a tolerance that the suppliers of the sensors work to, and that tolerance cannot/should not be excessive - be nice to know what those tolerances are )

At the end of the day, system stability is the most important (only?) factor in determining whether the OC is acceptable. Saying that, as many of us are pushing our systems to the limits, then extreme temps could severely reduce the lifespan of the CPU and other components.

Also taking into account that it is very difficult to get a reliably-calibrated temp sensor into a suitable position to record the accurate temps, and also considering that the CPU specifications are again based on tolerances - well, there are so many variables that it's difficult to provide an informed opinion.

At the end of the day, I would say that, if you're prepared to OC your system to the max, then component lifespan is not a major issue - just system stability. If you can maintain a stable system, then it's more likely that you will replace the components before they fail.