"straight perms" are my saviours and my worst nightmare in one. i have both used chemical straightening kits, and i've also done it for other people for a small fee, so i like to think i'm a semi-proffessional!lol
FACT- it damages your hair. is breaks down the properties that make hair curly. the extent of the damage depend on the aftercare. if you're going to chemically straighten your hair you MUST look after it properly! never let it become too dry, have regular trims, do not over-process it. THIS WILL continually weaken the hair and encourage breakage. straight perms therefore increase hair breakage.
look after your hair, and the chemicals shouldn't affect it too much.
if this is the first time i would recommend you ask someone with experience to help you/go to a hairdressers, ESPECIALLY if you have dyed hair.
saying all this, chemically straightening you hair can leave it silky smooth and saves you a fortune on GHD'S!
hope this helps!