If anything has been lived and taught by Warren Buffet it's this: if he doesn't understand a business, he won't buy or invest in it, simple as that.
The reason Buffett lives by that rule is because he is a long term investor, which is really the only kind there ever should be.
Warren Buffett, as someone said one time, read company reports like others read pulp fiction. Others have similar practices, and it really pays off in the long run, as the one with the most accurate information will win, as long as they know what to do with that information.
Finally never be MARRIED TO A STOCK...;)