What do you know about dry shampoo?

My gf asked me to track down Samy Dry Shampoo and Big Sexy Hair Dry Shampoo. Both seem to get middling to low reviews. Have you tried either of these or dry shampoo in general, and what do you think? I’m not sure why she wants it exactly, but her primary complaint is the thinness of her hair and everything she has to do to give it volume. Also, where’s a good place to buy it? Thanks.