Trying to buy an upgrade at embarkation seems dicey. If you're willing to pay, why not book what you want?

As for what you get as a repeat customer, you might get little, you might get alot depending on the cruise line and how frequent you are. At the very least, you'll get an "exclusive" cocktail party with half the ship who are also repeat customers,

Some provide neat benefits. Cunard for example extends a 10% discount to repeaters.

It's like the airlines, if you repeat enough, they'll be very nice to you.