I have never done this, but over on the
www.disboards.com, some people have, and others claim it is better to rent your points and then pay for the cruise. I think that the points can rent for more than the cash price of the cruise if you book early, so trading points for cruise can be an expensive way to cruise, IMO. Also, if you book the cruise with points, and have to cancel, you have restrictions on the use of the points after that. So, generally, I have seen people advised to rent the points that they would have used to book the cruise and use the extra cash for tips and excursions and so forth.
Others have just used points to book the cruise and said that the hassle was not worth it and just used the points for the cruise.
Either way, you are vacationing the way you want. You can book a holland america cruise to alaska or Disney cruise. I don't think there are any other cruises that you can book.
Hope this helps,
Tammy