I used lemon juice to lighten my hair when I was younger.  My hair was quite short (pixie) so the damage was not as noticable.  I would notice my hair getting straw-like, but that could also have been all the pool chemicals as well.  (I had 2 swim workouts a day - murder on hair!).  I wouldn't be willing to use the lemon juice on my hair now - too much at risk with hair as long as mine.