I have naturally frizzy hair... neither straight nor curly, but poofy, and at the slightest hint of rain or damp I turn into Hagrid. I straighten it mostly.
I don't spend much on myself but decided to treat myself to Keratin treatment.
It works. It actually bloody works! Had it done last week, my hair felt great and smooth if a little flat (they did the treatment then straightened it for me)
Walked the dog in the damp... no frizz. Got rained on today... no frizz... and post washing didn't need the GHDs, just a blast with the hairdryer and it's soft and smooth.
Just in case anyone is considering it... I think it's actually worth it!