I am personally considering a cruise next year. Pros? Cons? Who are the good operators and bad ones?
Pros
They are a lot of fun
You get to visit a lot of different places in a short period of time so it's good for a "taster" to see if you want to go back to that place
You possibly get to go to places that you otherwise couldn't afford (such as I went to Bora Bora and it was amazing!)
Lots of entertainment (though not always to my taste)
Lots of food
You can make some really good friends
You only have to pack/unpack once
Getting table service for every meal every day (we don't like buffets)
You get your bed made every day for you, and the sheets turned down before bedtime every night
Cons
I don't enjoy the sea days as much as some others do, frankly I'd rather be exploring a place that I've never been to than relaxing on board
There is waaaay too much food so it's very, very easy to overeat
The cabins are generally tiny unless you can afford a mini suite or bigger
If you are younger and going on a cruise, it can be difficult to find people your own age to talk to. We've been on a couple where we were the youngest on board by about 30 years!
Alcohol on board can be very expensive, though some cruise lines will let you take a limited amount in your luggage with you.
Good cruise lines: Princess, Royal Carribean, Holland America
Alright cruise lines: P&O (probably the most fun I've ever had on a cruise, but definitely aimed at the younger market. I would feel very old if I went on it now)
I can't comment on any other cruise lines as these are the only ones I've tried so far.