I am rather new to raising ducks (and geese!) but I can tell you what I have learned so far.
#1. They are MESSY! You will need to clean their pen/coop daily to keep down on the flies. If they range all day, that helps. They are messy eaters too, taking their food in their bills and then putting it in their water to make a soup and then eating it up sloppily! Have to say though, they are too cute when they do this, so despite the messes, they are still adorable.
#2. They get along very well with chickens if they are raised with them from young. I got all my ducks and geese as babies, so I can't tell you how adults would do with chickens.
#3. They are hilarious! They have such fun personailities and are playful and curious. Eventually they will pick a "leader" of their flock and follow this one everywhere it goes. They are fairly easy to handle and even a duck that hasn't been handled will usually calm down when you pick it up. I've trained mine to come to me when I call "Here duck ducks". They come running to see what treat I have for them, and they'll eat right out of my hands.
I have gotten week old baby ducks, and then a few weeks later got some slightly older goslings. They all accepted each other right away and get along wonderfully. The ducks are little hams, and do the funniest things. Its so cool to watch them dive into their pool and swim like lightening fast under the water. Then they'll pop up like a fishing bobber and do it all over again! They like to play with my shoelaces and chase our dog too! The "lead" duck even grabbed onto my dogs tail and held on!
Other than the messy part of keeping them, I highly recommend a few if you have the room. They are fun to watch, pretty easy to train, very easy to work with, and not much trouble at all. Just make sure they ALWAYS have water. Several different sources around the yard they range during the day makes this easy to do.