I've always found some of his advice interesting, particularly the borrower/lender idea. However many of his ideas are given an ironic slant with his last remark, which might seem to be violated by everything Polonius does:
This above all: to thine own self be true
And it must follow as the night the day
Thou canst not then be false to any man.
What he has told his son suggests he wants Laertes to put on a front (clothing) and listen more than he speaks. While this emphasizes the appearance vs. reality theme, making a good impression, because the company you attract and keep affects your nature, and being wise enough to know when not to speak are pieces of sage advice that Shakespeare would have known about from the Book of Proverbs in the Bible.
In another irony, you cannot be true to yourself if you attract to and surround yourself with false people and if you speak out arrogantly in a pretense of wisdom.
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.
There is a persistent idea (started by the Victorians) that Polonius is a bumbling fool. This doesn't work at all for me. Polonius is a highly intelligent political thinker. For example, his advice to Reynaldo on how to monitor Laertes's wild behaviour is magnificent and brilliant. His advice to Ophelia about love and sex is very accurate.
So, while waiting for Laertes ship to depart, Polonius has a couple of minutes to tell him how to live well. His brief advice is shrewd and parental. He tells Laertes to think before he acts, listen more than talk, keep good friends close but don't worry about drinking buddies. Don't get in fights, watch your money, take care of your appearance, but don't be a peacock etc. All sensible stuff.
Then he knocks out a Shakespearean pearl. "This above all, to thine own self be true." Most people seem to think 'to thine own self be true' just means 'be yourself' and obviously it does mean that. But I think Polonius (Shakespeare) is aiming much higher level than a trite 'hey... be the real you, kid'. He is really saying, "The person who lies to you the most often is... you. Be aware of the lies and distortions and half-truths that you tell yourself about yourself."
To thine own self be true. Don't believe your own propaganda. Rationally observe yourself and be on the alert for self-deceit. Become aware. A message that is usually too uncomfortable for most people.