We very much enjoyed our cruise on the Carnival Inspiration recently. I haven't cruised the other lines that have been mentioned, but we had absolutely no complaints, just rave reviews for the staff, the service, and most importantly, THE FOOD!
I did research quite a bit before we booked a cruise on which lines cater to which type of passengers. As a family with school-aged children, we were definitely looking for a line that was family/kid-friendly, etc. One of the neat things about cruising, IMHO, is that no matter from what walk of life you hail, there is a cruise ship to meet your needs. If you are a dedicated non-smoker, there's the Carnival Paradise. There are small ships that cater to a select group, ships that cater to adventurers, just everything - you name it, it's out there.
I think many of the complaints that people express about cruising, post-cruising, of course, stem from the fact that they may not have chosen the ship and/or cruiseline that best fits their expectations. In other words, if your idea of H$LL is a week with a bunch of kids running around, definitely don't choose Carnival or Disney during spring & summer breaks and holidays. That may seem like a no-brainer, but I can't tell you how many people who make that very choice and then moan about it post-cruise endlessly (duh).
Some books which were extremely helpful were "Cruising for Dummies" (hey, it worked for clueless-about-cruising me) and the Frommer's books on Cruising. They really broke down each individual line and detailed what each line is good at and not-so-good at.
To address your question, we definitely plan to sail on Carnival again, probably the Paradise next spring break if the finances work out.
Happy Sailing!
JO
We've been on several cruiselines, and Carnival is our favorite. RCI is very similar, but Carnival definitely has more fun! Our last RCI cruise (in October) was much too quiet for us and we can't wait to get back on a Carnival ship later this month.