The difference between males and females depends on the breed (other than the essential bits, obviously). Drakes often have a curly tail feather, and their quacks are usually softer and less expressive. In many breeds, they are more colorful when in full feather. They also, as someone already said, tend to be more aggressive, and you do have to be careful not to have too many of them per female.
Cayugas are gorgeous--deep black with an iridescent sheen. But again, you'll need females to round out your ratio.
The eggs are wonderful. They are richer than chicken eggs, and once you've had your own homegrown eggs you will never ever ever want to eat those tasteless pale things they sell in stores under the name of "egg." Duck eggs are larger, contain more of every nutrient (including, unfortunately, fat and cholesterol--but also including the good stuff), and the higher protein content means they cook up a bit stiffer and also give more lift to baked goods. I have served them many times to my chicken-egg-eating friends, and they have all loved them.
Finally, if you want pets, I highly recommend you get them as ducklings. They are not very expensive, especially if you can buy just a few ($4 or so each), and if you handle them a lot when they are babies they will be much tamer.
Good luck!
