My MIL has had it done - her hair looks fab. She has been on 2 hols to Spain and it survived the humidity - now been 2 months since she had it done and still looks much softer [and whispers] less wiry She paid £170 which included a cut and full size shampoo and conditioner.
I am thinking of getting an express version done at the Saks I go to. It's £90 to have the Keratin just blow dried in. But I figure I can then go home and get my own straighteners out and give hair a good go over. I have 3 colours incl a lightener in my hair and will have the keratin done immediately after my next root touch up. You'd have to wait a couple of weeks after highlights to get it done but it can follow highlights straight away (so says my hairdresser)