First, there are the temperature settings. Obviously, different hair requires a different heat level. The thicker your hair is, the higher level of heat you should apply to straighten it. I highly recommend you to read the instructions which usually say what temperature you should select for your particular hair type. This may vary a little bit from one flat iron to another. The general rule here is the less heat you apply, the better your hair condition will be. And always use a heat protectant, regardless of your hair type.
I read lots of different reviews about these straighteners and a lot said these aren't genuine. They are definitely ghds just professional ones which means they are lighter. They are exactly the same ones at my hair dressers. I'm happy.