I must say though, my bokken practise seems much more valuable than my jo practise, for developing empty handed technique. [/b]
I don't disagree with the value of the bokken, but one aspect of jo work I find very interesting(but also quite frustrating) is the practice uke and nage can do with one jo. Obviously, one can't actually do bokken work with uke holding the blade and nage holding the hilt, but this is possible with the jo. Since the connection between uke and nage is solely through that slim wooden stick, effective movement is much more graphic; any "wrong" movement will immediately be felt as resistance, which can't be overcome by merely being stronger than your opponent. Maintaining the feeling of having your center and uke's center connected is quite difficult, but is a very good way of testing and improving empty-handed technique.