In my family, the Disney cruise line is considered "the" premier Disney experience. While WDW is a must every year, the cruise line is something we absolutely believe Disney shines at. Not only are you surrounded by the magic and wonder of Disney (pun intended) basically 24/7, with the exception of the few hours you spend on shore at the ports of call, but the overall experience is simply phenomonal. If you love WDW, you will have an absolute blast on the cruise line.
You will notice that a Disney cruise is pricier than many other cruise lines. You must be careful when comparing though because, for instance, a Carnival ship is not equal to a Disney ship. While there are ships out there that are grander and more elegant than the Disney Magic and Wonder (particularly the newer Princess ships which I had the opportunity to sail on a few years ago), everyone in my family believes there is nothing that comes close to what the Disney Cruise line offers in entertainment, particularly for our family. That "Disney Difference" that Disney talks about in their marketing materials all too often is absolutely evident on the cruise ships and they really do stand out among what other cruise lines offer nowadays imo.
Of the 7 day Carribean cruises Disney offers, the Western Caribbean is my personal favorite because I believe the ports of call are better than those on the Eastern. If you are planning a cruise for this summer, the Disney Magic will be in the Mediterranean and during the summer of 2008, the ship will be on the west coast for a yet to be announced itinerary. It sounds like it will be a Mexican Riviera cruise once again, but the actual itinerary should be provided later this month.