Here is what you need to know. Those certificates are not "free" but will give you a heavily discounted cruise. You will pay something called a NCF, and so called "port charges" for all the cruises - these run around $300 for a Caribbean or Mexican cruise per person and about $400 for Alaska per person. Plus you will pay $12 per day per person gratuity and any cruise excursion, alcoholic or soda costs, other onboard ship costs such as purchases of souveniers.
To try to figure out if it is worthwhile, you should contact your host and try to find out if you have an offer for a Inside, Oceanview, or Balcony cabin and what itineraries are available. Usually your cruise offer states what kind of cabin (you should look at it carefully). Sometimes a host won't know which itineraries are available, but probably they include Caribbean and Alaska at a minimum and may include Mexico, Mediterranean, and more.
The cruises usually are 7 days (some low level offers may include 3-4 night cruises).
If you receive an Inside or Oceanview you can pay a few hundred extra and upgrade to a Balcony. Cruises are for 2 people, if you want to bring a friend or two or family member in the same cabin you can pay extra for them.
If you assume you will pay $300 for those NCF and port fees, you can go online and look for a comparable fare for a NCL ship at a site such as Travelocity. Take for example a typical Caribbean cruise you might be interested in. If Travelocity shows an Inside cabin at $599 per person (plus gratuties which you must pay at time of sailing both through a NCL certificate or through Travelocity), and you are paying $300 from the certificate, you have a 50% savings. If you are getting a Balcony cabin to Alaska, and that cost is listed on Travelocity is say $949 and you'll pay $400, you are getting more than 50% savings.
All in all thinking of the certificate as a 50% savings is probably a good idea, but it could save more for an expensive trip.
Now, if you would go on an NCL cruise if you could get a 50% savings - then go to get the certificate.
For single people -- one person can use a certificate by themselves and pay just one "fee". This can be a lot cheaper than booking through a normal channel, because usually singles have to pay for a double cabin and pay the charges for two people. So a $600 base price for a single person becomes $1200. Now the $300 in "fees" you pay for a single person becomes a really significant savings.
In the case of having to fly somewhere to get the certificate, maybe it wouldn't be worthwhile. If you have to spend $500 per person to fly to Vegas (and wouldn't normally want to go to Vegas), figure $500 plus at least $300 for you basic cruise "fees" - and now you could spend $800 and go on Princess or Royal Caribbean or anywhere else, not just on NCL for your cruise. (And NCL isn't the best cruise line).
On the other hand, if you would like to vacation in Vegas, spending the $500 on the flight doesn't matter, so the cruise certificate (whether you use it or not) is just a bonus, so go for it, and if you like the destinations and pricing you can use it (or not).
Note - you will have to book the cruise within 2 months of getting the certificate or the certificate voids out. You can cancel the trip usually up to 2 months prior to the trip, so if you get a certificate June 15, you might be booking a cruise for Dec. 15. You pay those "fees" before Aug. 15, but could cancel by Oct. 15 (maybe even later) if you had to and get back the fees.
If you have never cruised before you will greatly enjoy the NCL cruise. If you have cruised before, NCL isn't the best cruise line. If you have cruised on a lot of NCL cruises (as we have) you may not be all that entranced because the food and entertainment tend to be very similar on all their vessels.