We got our first MBNA card 3 or so years ago. I now have 5 so I can maximize my points. We have used points on 4 cruises, which have been free or nearly so.
The certificates are usually easy to redeem although I have had times that it took 6 weeks to receive them because they lost my request. You can ask for expedited service for a fee of $35 per certificate (they only come in $500 denominations). When you mail them to your TA or NCL be sure to send them Certified Mail, return receipt requested because if they are lost they are the devil to replace. Also make a copy of your certificate prior to mailing.
You can use them for any air, hotel or transfers associated with your cruise IF you book them through NCL or your TA.
They have to be used in even $500's. Meaning if your cruise is $1,200 you can use one or two certificates and pay the balance or you can use three certificates but you will lose the excess $300. They will NOT apply it for OBC (rats).
Certificates are good for one year from date of issue so let your points build up if you're not cruising often. Although, points themselves do expire 3 years from accrual. (But who can wait 3 years to cruise!!!) Points accrue on a non-calendar year (starts with the month you receive your card) so you need to keep up with your dates and points since after 1,000 points you don't get a thing more til your anniversary rolls around. That's one of the reasons I have numerous cards. 5 cards = $5,000 of sailing. Yahoo!!
If you have any more particular questions, just ask cause I've probably been throuh all the weird situations with these certificates you can imagine.