i had mine done 4 years ago and im planning on getting it done again in winter.
It was a godsend!! my hair was just longer then shoulder length and it took about 6 hours. I previously had blonde hair from 1999-2003 and went back to my natural colour of dark brown. i had my hair done in 2004 so i was still growing out the blonde hair with the dark brown dye over the top.
So i had virgin brown hair maybe up to my ears, from ears down still obviously had the bleached hair with just the brown on top. So my hair was sensitised.
After i got it done, i could walk out of the house with wet hair and it'd dry straight like i'd straightened with it my GHD hair iron. Best money i ever spend. I never got the regrowth done cos my hair starts to get curly after my ears. So i had a good 14 months before regrowth was even noticeable. Even then the length and weight of the straight hair pulled the curls straight.
Just make sure your HD stress tests your hair. it needs to be strong enough to withstand being taken to breaking point.
The first cream they put in your hair breaks the curly hair bonds. They basically have to take it 1 step before your hair turns to jelly. They need to do this quick enough and then be able to rinse it out. Rinsing it out takes a good 20 mins because they need to get all the cream out of ur hair.
What they do next is blow dry it straight with a brush. Then the next bit is what takes the longest. They take tiny little pieces of hair (think 0.5cm thick) and iron over it at 5 different pressures to make sure the cuticle of the hair is straight.
Next they put the solution in with seals the straight bonds in your hair. Wash it out again. Finger dry and its dead straight!!
You cant place your hair behind your ears for 3 days, or wash it as it is still oxidising with the air. If your hair kinks or gets messy you can quickly brush it out straight with the hairdryer. On the 3rd day i came in for a treatment, cut and restyle for free.
Its risky but its worth it. Make sure the HD is a certified Bio ionic technician, even ask to see their certificate of training.
good luck
let me know how u go!!