Chemical straightening can thin out hair, make it dry, and make it limp and lifeless. Most people that I've known who have used damaging chemicals in their hair have found that it can never be restored to how it was before they started using chemicals on it.
I should think it depends on your hair type, though. Very very thick hair may not take such a blow and may not be as thin, whereas hair that was thin to start with may become straw-like. At least, in my experience, this sort of thing has happened.