The DVC preview is basically a chance to hang out with other DVC folks. There is some free swag, some prizes, some fun, usually some drinks and snacks. My son got a coloring book. The special guest is usually a character, I like when Chip and Dale come. Those guys are the best. They will sometimes also have one of the variety acts come and perform if it is a longer cruise. The discounts aren't anything better than what you get on land. Though I think if you buy like 1,000 points there is some nice looking totebag they'll throw in. My recommendation if you are serious about adding on direct from Disney, go onboard with the current land discounts and point amounts and then price shop. But the days of discounts at sea has gone away.
Also, if you stop by the DVC desk once a day they'll usually hand you some sort of swag, like a ppicture of walt and lily on a steam ship, light up bracelets for pirate night, etc.