its quite a harsh process apparently. the stuff they use contains lots of chemicals including those found in perm lotion, and its very drying and damaging to your hair.
are you wanting to go from dark to blonde?? if so, see your hairdresser id say... it can be done at home with a pre-lightner then normal hair dye. but... to be on the safe side, hairdressers would be best.
another thing to try if youve dyed your hair and think its too dark, is vosene shampoo. it stinks, but it lightens hair.