Pros
You will learn a lot about persuasion, public speaking, and time management. Beyond that, you may get to see some amazing destinations, hang out with co-workers from around the world, and immerse yourself in the cruise lifestyle! There is a lot of fun to be had, and when the sales are good, it's all the better. It is a very unique job, with memories to last a lifetime. Also, you can take 6 month contracts, and then vacation for 2, and back and forth, receiving your pay all along!
Cons
Your experience will depend on your ability to adapt to new situations, and be very dependent on your boss and teammates. It's a small ship, you see each other all day, everyday. You may get someone really intense and/or hurtful. It's very high-pressure sales, and some people crack. Don't expect sympathy. Also, unless you are from outside the US or Canada, you pay could be really crappy, so it's up to you to decide if food & board/the experience is worth it. And sometimes, depending on what you are selling and to who... you feel pretty terrible about yourself.