Unsweetened fruit juice is still high in sugar comparable to soda levels, and high in fructose which is particularly bad sugar to be high in.

Sugar levels in juice compared to Coca-Cola: http://www.hookedonjuice.com/

Dr Davis' Goodbye Fructose: http://heartscanblog.blogspot.com/2009/07/goodbye-fructose.h...

Dr Robert Lustig's what happens when we eat fructose and why it's bad video: http://www.youtube.com/watch?v=dBnniua6-oM

Aside from the hydration, you're probably better off with even sweetened coffee than unsweetened fruit juice.