I bought the generic Batiste original dry shampoo (the green one) with me to try out.
When you spray it, it covers your hair in a fine mist of white powder which I guess absorbs the grease n stuff from your hair. The next stage is to rub in the powder so it is no longer visible. This is where there is a problem. It is not easy to stop the powder from being visible in your hair. In the end, I found that you end up with slightly faded roots which become greyish in colour so you sort of look like your going grey. Also if you spray too much some of it will fall out your hair and you will be covered in little white flakes The bottle claims it will "boost body" but don't believe this. What is claims is added volume is actually just excessively frizzy hair. However, although it replaces greasy hair with frizz, it does banish oily roots and depending how greasy your hair was to start with, this is usually a slight improvement.
|greasy hair before|
|right after spraying|
|partially rubbed in|
|fully rubbed in|
Overall, using a dry shampoo provides a slight improvement on greasy hair, but unless you are going somewhere where you can't wash your hair then I really can not see why anyone would want to use it. Maybe the colored versions are better, but then so is actually washing your hair.
Festival world yes, real world not really.
Heres a more postive review of dry shampoo, here is a review of coloured dry shampoo