I think it is impossible to make a good recommendation to someone about a cruise line without knowing something about their likes/dislikes. My family of 5 won't even consider Carnival or Norwegian, but strangely, love Holland American (a line considered to be for "old people" by many).
The suggestion to read the forums a cruisecritic.com is excellent-- lots of info there. Reading the individual forums for each line may give you a feel for what people similar to yourself have experienced on board.
1. How much is realistic to pay for a cruise that lasts 4 or 5 days?--We recently paid $399 per person (double occupancy) for a 7 night Holland American cruise out of Fort Lauderdale. Putting a 3rd and 4th pax into the same cabin would have been $99 each plus port taxes of $45 per person.
2. Which cruise lines give the most bang for the buck?
This is an impossible question without knowing what you consider to be "bang"-- I value good food, good service, quiet atmosphere and HATE constant attempts at selling me crap. I also hate a "rah- rah" atmosphere that some other people love. What I consider ideal, you may consider boring and lifeless.
3. Which are the best as far as dining and fun things to do? (we're in our 50's, our kids are teens and low 20's). There are constant arguements about which line has the best food-Even within the same line, I have found that food quality varies from ship to ship.
4. Which cruise lines would you stay away from? I wouldn't even consider sailing on Carnival or NCL. I have considered Royal Carribean for trips with the kids (because of the wide range of kid friendly on board activities), but haven't sailed them yet. I currently consider Celebrity, Holland America and Princess to offer the best balance of price/quality for my family.