Hey Jake,
I went on the Destiny in Feb, 2005 with some college friends. At the time, we had been out of school for about a year and we were all around 25 years old.
The boat and itinerary are fantastic. The downside to this ship is that most passengers are Spanish speaking. I would say that 75% of the boat was Spanish speaking and were natives of San Juan. Most of the music in the bars/clubs was Latino as well. This might be what you are looking for but for us it was difficult and frustrating to communicate/connect with many of the other passengers.
I have been on 8 Carnival Cruises and have always been the one to make the arrangements for our group. It’s a lot of fun and can be rewarding for you if you hook your friends up with a good deal and everyone has a great time. Here's my advice:
If you are flying out of New England, plan to arrive the night before to avoid being delayed and missing your sailing do to a snow storm. It almost happened to me so I always fly in the night before. As mentioned above, take into consideration the extra cost of lodging when comparing cruise prices. Compare different cruise itineraries based on the total cost per person including air, hotel, transportation, parking, etc.
Take a direct flight- No connections!!! I went on a cruise out of San Juan (Royal Caribbean) and had a connection in Philadelphia. The airline lost our luggage and it took 3 days for it to arrive. It was terrible! You and your buddies will have a difficult time picking up the ladies if you’re wearing the same t-shirt on day 3 that you wore the day you boarded the ship- trust me -

. This can be avoided my booking a direct flight. If your buddies are hesitant b/c they don't want to spend the extra money for the direct flight remind them of what can go wrong.
Splurge for a balcony. Great place to have a few drinks before heading out and even better place to end the night once the bars have closed. Plus, the ladies always enjoy a cocktail on the balcony.
If you are on a tight budget go for an inside cabin- do not spend the extra money for a window as they are useless. Either go inside cabin in the bottom of the boat to save some money or go big and book a balcony.
Alcohol- Read the numerous threads on how to smuggle it aboard. This is a great way to save money and will allow you and your friends to get a buzz going before you get up on deck or go out for the night.
Request the latest Dinner seating available. Less families with children and it gives you some time to get a nap in or catch a little extra sun before dinner.
Let me know if you have any questions. Good luck in your planning.
Louis