I'm an ACE-certified personal trainer and group fitness instructor who teaches juggling for fitness (JuggleFit). Juggling is certainly a workout and can burn up to 280 calories an hour. I would recommend doing it for cardio for 30-45 minutes 4-5 times a week.
I recommend against using weighted balls as that can be bad for your wrists and cause injury. Reps with dumbbells are okay because the weight is not falling into and impacting your hand/wrist - it's a slow, smooth and controlled movement. But since juggling is an endurance activity, and constant, the repetitive stress is dangerous. Many have warned against using hand and ankle weights while walking for similar reasons.
Juggling will give you some muscle tone in your arms, but if you want something more, I suggest creating a circuit workout based around juggling. Set a juggling goal, whether it's 5 minutes of continuous juggling, or juggling in a new pattern until you drop. Then do 10-15 reps of lunges, squats, pushups, etc. Then back to juggling. Create a 30-45 minute circuit of juggling with upper and lower body moves as mentioned.
One more thing that is great - if you juggle 3 well - hold a lunge for 15-30 seconds while juggling. This really fires off your leg muscles and engages your core. You will feel it!
Check out Benefits of Juggling for many ways juggling can benefit your body and brain.