My mum had it done. it was several years ago, maybe 5 years, so perhaps the technique has improved since then? Anyway, the result was not what we expected, which was soft, silky dead straight hair! It was certainly straighter, but she still needed to blow dry/run the straighteners over her hair to get a really straight and NOT frizzy result. Left to dry naturally her hair was quite frizzy even after the chemical straightening. But the worst thing was the damage it did to her hair, which was dry, brittle and snapping off underneath! And, yes, she did go to a reputable, upmarket (and expensive) salon. They warned her that it might dry her hair SLIGHTLY, but not to the degree she experienced.

So... I wouldn't have it done, but again, perhaps that technique has improved over the past few years!