Fungo - Good topic. It will be interesting to read the responses.
As I look back I can think of many, MANY examples of GREAT advice. Some from members of this message board.
Most of the good stuff we were lucky enough to have followed...but not all. Some of that GREAT advice came from all of the types of people you listed above.
We also, obviously, got some not-so-great (ok, "bad") advice. Some of it had to do with which summer team to play for...but a lot of not-so-great advice on some of the colleges that recruited our son. As one example, quite a few people told us that the school our son ultimately attended overused pitchers (mostly related to John Hudgins' marathon performance through the CWS). Much of that advice came from competing schools. It has been quite interesting to me over the first 2 years to see those same schools who asserted the oversuse themselves "overuse" their own pitchers...i.e. bring their Friday starter back for a Sunday closer role after a 100+ pitch appearance on a Friday. That to me is FAR worse than starting on 3-4 days rest in the CWS. At the same time we have found that his school protects their pitchers FAR MORE that he was protected in HS or summer ball. So I'd put this in the category of "bad" advice that was given to us. It has worked out the opposite of how we were told.
Its important for all of us to realize that the source of the advice has an agenda. Heck, when I speak I have an agenda, you have an agenda, everyone has an agenda. We ALL have an agenda. We just need to understand that "advice-giver's" agenda and then pick out the parts that are wise and useful and discard the parts that are in his/her best interests if thats all they are.
Hard to do this? Yes, sometimes. But a little thought, research and contemplation is worth a lot more than an emotional quick decision. Thats my "advice."