Just got back from our most recent cruise on Sunday. Had a blast!
We've done three cruises--our honeymoon seven years ago on Royal Caribbean (stops in Ocho Rios, Jamaica; and Labadee, Haiti--the latter of which was a hurricane-forced detour from Grand Cayman), last year with our three kids on RC (stops in Grand Cayman and Cozumel), and the recent one with the three kids on Disney (stops in Nassau and twice at Castaway Cay, Disney's private island).
Our opinion--if you have kids, plain and simple, do Disney. The difference between them and the rest of the pack for all ages of youth is light years apart, and easily worth the cost increase.
If you don't have kids, or you do but aren't taking them with you :banana:, Disney was still slightly better than RC, but not by a big enough margin to justify the price difference.
I disagree with the advice about "always get a balcony". You're in your state room basically just for sleeping--otherwise, you're either elsewhere on the boat or ashore. Why pay extra for a feature you'll use so little.
I agree with avoiding the generic tours--but if you want a specific type of excursion (snorkeling, parasailing, fourwheeling, whatever), you'll probably be best off booking an excursion. But you DON'T need to book them through the cruise line, and in fact can often find better deals through non-affiliated websites. Note, however, that if something goes wrong on an excursion and you're delayed, most boats will wait for you if you're on a ship-coordinated excursion, but NOT if you went outside of them.
I had good luck booking my first two cruises, comparing costs and features of different cruise lines and boats and ports, finding the best deals, etc., through www.vacationstogo.com. But don't just rely on the online info when booking--call them up and talk to a human. If you want more on Disney, a good agent I worked with was www.smallworldvacations.com.
You definitely want trip insurance for a cruise--I used travelguard.com, seems to have the best rates, especially if you're traveling with children. And even if you're told you don't need a passport, go ahead and get one--if something goes wrong and you have to fly into the US (rather than boat in--examples are ship problems, missed getting back on at port, serious injury/illness, etc.), you're screwed unless you have a passport.
Most importantly--HAVE FUN!