I'm thinking of having my thick, wavy hair permanently straightened. 
But friends reactions are along the lines of 'don't do it - it ruins
your hair'.  It's difficult to find objective advice on this subject. 
Does it ruin your hair? Which method is best?  What about regrowth? 
And does it really damage  your hair badly?

I have an abundance of naturally curly hair. Back in the 1960s, when
stick-straight hair was in fashion, I straightened my waist-length
hair chemically. The straightened hair was much less shiny than my
natural hair, but it certainly wasn't "ruined." I had to re-do the
process about every four months because of curly regrowth.

One good thing about hair is that, no matter what you do to it,
eventually new hair will grow back. Unless you suffer from poor
nutrition, any damage that is done is not passed on to the next crop
of hair that will sprout from your scalp.

Back in the 80's I had my hair straightened and I was surprised at how
soft and smooth it felt!  I had it done in a salon by a person who was
familiar with my hair.

Back before Pink's time, on the East Coast the girls ironed each
other's hair to straighten it.  Looked good, can't remember if I ever
discovered that it was soft.
(Did they have to use starch?  And no, I can't recall any scorched blondes.)

Hi, I actually read this really good article about permanently
straighten, you'll find more useful information there I'm sure:
http://www.carefair.com/Hair-and-Nails/Hair/How_to_Permanently_Straighten_Curly_Hair_157.html