Keratin conditioning treatment - does it damage your hair? Does it always look great?

[Kalypso]

I am fed up with my hair.

It's mostly straight, long, very fine and tends to kink/wave at the sides. I have little short bits of fluffy hair at my temples which never grow and curl up like little horns in humid weather. I can load my hair with products, but it never stays glossy. At the first hint of moisture (whether in the air, or sweat), it frizzes up again. I don't tend to straighten it because I'm concerned about damage.

Sooo...I am thinking of having the Keratin conditioning treatment, as I want my hair to look nice for my SIL's wedding in the second week of October. However, I can't really afford to have the treatment more often, so it's likely that it's going to be a one-off, at least until I go back to work and earn some money.

I'm a bit concerned that when it wears off, my hair will be more dry/frizzy afterwards? I admit I know very little about it.

Secondly, is there any chance it could actually look a bit rubbish? My hair is really, really fine, and I'm concerned that while it might look super smooth and glossy, it will also look really flat and plastered to my scarily small head.

Can anyone offer any thoughts/suggestions?

Thanks in advance!

[RamblingRosa]

In my experience (I had it done about 3 times) it did take its toll on the condition of my hair. They say it's good for your hair but my hair was definitely drier and more fragile and brittle afterwards.

I'm also put off by all the stuff about the levels of formaldehyde (even the ones that say they're formaldehyde free) and how it can be carcinogenic and cause skin reactions.

To be honest, it didn't have an incredible effect. My hair was easier to blowdry and resisted frizz better but it still looked frizzy/curly if I just washed it and didn't blow dry/straighten it. Seems like a lot of money to spend on a treatment to then still have to blowdry and straighten hair!