Pros:
If you get a good egg laying breed (Khaki Campbells, Welsh Harlequins, Runners) they will outlay your chickens. The eggs are jumbo sized and are excellent for eating and baking.
They are more naturally disease resistant than chickens and are extremely cold hardy. On days that my chickens refuse to leave the coop (-20 degree temps) my ducks were outside swimming in their water bowl. When it gets cold, my chickens almost completely stop laying eggs. My ducks keep right on laying.
They are so much fun to see waddling around the yard. My Mallard also flies around the yard.
Cons:
Boy are they messy!!!!