Is Polonius a fool or a wise, long-winded man in Hamlet?

In Shakespeare's "Hamlet," Polonius seems at times misplaced: a stock comic character in the midst of a tragedy.  In the serious scene between the queen and her son in Act III, Hamlet stabs at Polonius, believing him a spy.  When he lifts the arras and sees the body of Polonius, Hamlet cries,

Thous wretched, rash, intruding fool, farewell!/I took thee for thy better. (III,iv,32-33)

As he leaves the room, Hamlet drags Polonius behind her, telling his mother,

...This counsellor/Is now most still, most secret, and most grave,/Who was in life a foolish prating knave. (III,iv,217-219)

That he is a "foolish prating knave" is evidenced in the treatment of his children by Polonius. With them he is base, hypocritical, and dishonest.  When Laertes comes to his father before he departs for France, Polonius spouts platitudes such as "To thine own self be true" (I,iii,78), and, then, after Laertes is gone, Polonius sends a servant to spy upon him.

As the chief observer and part of the corrupt court of Denmark, Polonius lies to his own daughter, Ophelia, telling her that Hamlet does not love her so that he and Claudius can secretly observe Ophelia and Hamlet in order to prove to the king that Hamlet is insane in hopes of gaining favor with the new king.  Encouraging Claudius in his nefarious ways, the sycophant Polonius makes a swift transition from one leader to another, telling Claudius in a long-winded speeech that he "will be brief,":"Your noble son is mad"(II,ii,92). even the queen senses his foolishness, demanding, "More matter with less art" (II,ii,95)

The aged minister of state is, indeed, a fool at times, long-winded at others, and a hypocrite who represents the corruption of the court of Denmark.  Yet, he proves himself unwise since his subterfuge costs him his dignity as well as his life.

In Hamlet, was Polonius a foolish old man?

I find Polonius a bit foolish.  He was a a busy-body, which means that he was into everyone's business, meddling in affairs, and projecting his own opinions on issues all of the time.  He gives his son, Laertes, a bunch of long-winded advice about life and living successfully, which was delivered at such length that it was a bit tiresome.  Foolish old men like to think that they know more than everyone else, and are constantly dispensing advice, whether it is sought after or not.

Polonius is also constantly getting into the king and queen's business, in an almost "suck-up" sort of way.  He takes information given to him by Ophelia, about Hamlet acting strange, and goes and blabs it to the king and queen.  Then, he manipulates a meeting between Ophelia and Hamlet to please the king and queen, and help them in their quest to determine what is wrong with Hamlet.  How would you like it if your dad found out that your boyfriend or girlfriend dumped you, then went to their parents and set up a secret meeting with you and the ex-boy/girlfriend just to see how everyone would act?  Very meddling and embarrassing indeed.  So, in that, I feel that Polonius was a bit foolish.   In the name of sucking up to the queen and king, and of being a bit of a gossip, Polonius kind-of throws his daughter at the whim of royalty.  I find that foolish and rude.

Then, Polonius foolishly hides himself behind a curtain while the queen and Hamlet have a private conversation.  Yes, that's a bright idea, isn't it?  How annoying, and why is it his business anyway?  Granted, he didn't deserve getting stabbed for it, but, if he hadn't been foolishly putting his nose into other people's business, he would have never been there in the first place.  So, for those reasons, I find Polonius a foolish old man who talks too much, meddles too much, and contributes to his own demise.