Wow! Some positive reviews here!
I, on the other hand, have a bit of a different view.
I have softly wavy hair and was advised by my salon to purchase GHD straighteners with kerastace protective products a few years back.
I can honestly say that even after only using them on average twice a week, my hair is literally ravaged to death by them, regardless of their shining reviews. I noticed the difference around the 2 month mark, my hair became extremely brittle and so dry at the ends, more porous, frizzy, less responsive to conditioning and growing much slower.
My technique was to allow the hair to dry overnight then straighten in the morning, or else blow dry first then lightly smooth the irons over (just the once!) to finish off.
No matter how careful or minimally i use them, they have literally torn my hair to shreds. I followed all of the advice i was given, and i really do think we are being fooled. I was also told that my hair is pretty strong, considering, so it was not already weakened prior to using the irons, nor has it ever been coloured.
It also straightens pretty easily, so i wasnt even going over the top!
I am amazed that so many people swear by this stuff.
Since i quit using the GHD's and reverted back to mild heat blow drying, my hair has finally begun (after a few months) to respond once again, and to seem at least partly alive. No going back to that!