So you are curious about the Brazilian Keratin Treatment?
The process costs on average $250. It is temporary. It lasts about 2 to 3 months on average.
In a nutshell, a 450 degree flat iron infuses a keratin treatment/solution into the hair and seals it in. Certain shampoos can raise the cuticle and cause the treatment to release faster. Shortening the longevity of the straightening results.
I personally took a Rejuvenol class on it, but did not buy the intro kit . . . mainly because people complain about paying $80 for a relaxer service that lasts for 2 months. I don't even want to offer $250 process that lasts for 2 months.
As far as health of the hair; I have not seen it to be any healthier. It is more a styling enhancement for those who shampoo more than twice a week at home.
Some BKTs have formaldehyde. Some do not. Typically, the ones with formaldehyde last longer.
Clients are forced to wear the sleek or bone straight look all the time. So if you want to have more curl or body without the frizz, please consider using higher end shampoos and treatments such as Pureology and Aveda. Your hair will transform over time with higher end products.
If your hair is straight, wavy, or slightly curly naturally, then the BKT will be a great option for you because after about 3 months your unprocessed/new growth has never been unruly, but you desire more smoothness. The BKT would avoid any permanent processing of the hair, but the sleek/straight look should be your desired style everyday. It will make your regular home maintenance easier.
If you are overcurly/kinky naturally, then after six to eight weeks, your new growth will still be the same. So you will have the choice to BKT it again at the $250 rate or go back into the relaxer touch-up for less.