We have been on 4 Disney cruises (5th in August) and 2 Royal Carribean
Always sailed from Port Canveral for the convenience, its with 1 hour from the parks but their are other ports and other ships to look at.
Depends what you are looking for but Carnival tend to be more "fun" cruises, we have frineds who didnt like Carnival but they have young children. Said it was all drinking and gambling so maybe more suited to adults.
We like both Disney and
Royal Caribbean. Don't think Carnival is for us.
Other options would be HAL, Holland America or NCL, Norwegian, both sail from Florida ports.
If you are not tied to a cruiseline I would look at the offers, for eg I keep looking at Alaska for May half term, HAL we can get 7 nights for about £300 each
RCCL is about £500 each. both from Seattle.
For a good online priceguide try cruisedirect.com I used them for our RCCL cruise last August. They send me an email every week with latest offers, some are mega cheap, one this week was on NCL, 7 nights around Carribean 28th March 09 £436pp Same cruise booked on thecruisestore.co.uk would be £662pp even with the poor £ against the $ booking in $'s with a US agent still seems the best option.
Some prices are available on cruisedirect.com for 2010, even the odd one for 2011.
Shout if you need anymore help.