At karate class we just had a special seminar on the basics of using the nunchaku. Wow! Talk about a challenging weapon. And I thought tonfa was difficult to learn. At least when learning to use tonfa you don't normally run the risk of accidentally knocking yourself unconscious. But that is a very real risk with the nunchaku if you aren't careful. The seminar was a lot of fun, albeit a little bit painful at times. Everyone I talked to afterwards was happily describing the various self-inflicted injuries they received. (I whacked myself in the side of the head once and hope to never do it again. It hurt quite a bit and I still have a small lump from it.) I don't know of anyone who didn't accidentally hit themselves at least once. It gives you a good appreciation for how dangerous these things can be and how careful you need to be when practicing with them.
Another very serious consideration with these weapons is their legality. Depending on where you live, they may be illegal. I know in Massachusetts they are illegal, so to my young friends in Boxford i must say don't even think about it! In Rhode Island there is a statute that allows the use of martial arts weapons for education purposes. Actually, the statute allows the use of any weapon for educational purposes, not just martial arts ones. The statute (11-47-43) is an exemption from the provisions of statute 11-47-42 which specifically forbids the use of "the so called 'Kung-Fu' weapons", along with a long list of other weapons. (Yes, the state law really refers to martial arts weapons with that exact phrase - the so called "Kung-Fu" weapons. Pretty funny...) So, at least where I live I know its OK to have these weapons for karate class. Just to be safe, I always carry a recent receipt for payment for my karate class and I carry a copy of the RI statute number, and I only carry my karate weapons in my gym bag with my gi and I keep it out of reach in the back of my car, so it's clear I'm not planning on using them outside of class.
Of course, I'm not a lawyer, so don't base your actions on what I describe here and assume everything will be ok.