I contacted David's about this and will likely use them to book my cruise. In short, they are eliminating a step by booking the cruise for you instead of sending you a check for renting out your points. They look into the cash amount the cruise would cost and then convert that into the number of points you need to give them to rent. The point savings will obviously vary upon what you choose (room type, category, part of the ship, length of cruise, which ship, and so on) so you need to check with them directly on your particular cruise. On one example I asked about, the cruise would have cost me around 300 points booking through DVC, but using David's would only cost me around 200 points. They told me another example where someone this morning saved 54 points on a cruise.
So it looks like if you are the type that wants to make your own reservations and control the process, then you probably wouldn't want to do this. Instead you would rent out your points and let them send you the money. If you don't mind them making the reservation for you, then their offer is something you should consider.
