Jason Kottke had a link to this article on why you should juggle. I couldn't agree more. Juggling is one of those things that I feel has contributed immensely to my physical and emotional capabilities and, to a lesser extent, my intellectual.
I learned to juggle when I was in ninth grade. I was in a self-taught mathematics class and a number of us flew through the material and wrapped up all the required stuff with weeks left in the school year leaving us with an hour of free time every day.
The father of one of my classmates was a professional clown and my classmate taught me how to juggle, first by juggling beanbags off of walls. I still maintain that this is the absolute best way to learn how to juggle. I don't quite know why but it is much easier, and those I have taught to juggle agree, to get the rhythm by first doing it off a wall.
Since that time, I juggle frequently. Pretty much whenever I find myself with a trop of objects. I'm not that good and cannot handle more than three but it is a lot of fun. It alleviates boredom. I don't know how often I have waited for a ride or to meet with someone running late and I grab three rocks or pinecones or something and juggle. I used to keep a bag of baseball equipment in my trunk so I could juggle (not only three balls but the always challenging bat, glove and ball).
Where it has helped me the most, though, is improving and encouraging further improvement in the symmetry of my body. Juggling requires using both hands. I used to be predominantly left-handed. As I juggled throughout the years, my coordination developed in my right-hand. When I played baseball, I switch-hit (albeit poorly). I extended that to golfing, hockey, and racquet sports (especially ping pong, where I enjoyed whooping on my friend Eric with my "weak" hand). I write all right with my right-hand.
From there I started messing around with my feet, too. Then once I got into martial arts, my ambidexterity really took off. Many people think my dominant leg is my left even though I'm "naturally" right-legged. I've reached the point where I don't really have a side of my body that is dominant.
Granted, it has been a lot of work in a lot of different ways that has enable me to achieve this but there is no way I would have even begun down this path had it not been for juggling.
As for the emotional and intellectual side, as I said, it alleviates boredom. There is probably something to the whole hand-eye thing, too, and the brain but I don't know that I feel like juggling has benefited me in that area as much as the physical side.
Definitely take the time to learn. It's fun and it's good for you.