Since parts of Polonius' advice has been quoted for centuries and several lines has become proverbs for good behavior, I would say his advice was very good. We often hear the following adages, not even realizing they were originally a part of Polonius' speech to Laertes:
"For the apparel oft proclaims the man"I,iii,76)
"Neither a borrower nor a lender be;" (I,iii,79)
"...to thine own self be true," ( I,iii, 82)
In addition, other piece of advice are often said in other ways, but are still part of Polonius' advice.
"Those friends thou hast, and their adoption tried,
Grapple them to thy soul with hoops of steel" ( I,iii,66,67)
"Give every man thy ear, but few thy voice;( I,iii,72)
Although Polonius was rather addled in some of the scenes in Hamlet, this speech survives as some of the best advice a father could give a son.