Yes, paintballs do hurt, but it's a very short lived pain. This pain can easily be avoided by wearing the proper protective clothing while playing paintball.
Yes. Paintballs are actually not paint at all, they are a clever mixture of detergent, and coloring. Paintballs are made to wash out of clothing, so even if you accidentally wear a nice piece of clothing, no worries, it'll come right out.
A player will go thought an average a 100 rounds of paintballs in about a half hour or less. Groups with 10 or more typically shoot about 200-500 rounds per person in a given 2 hour peiod of time.