@turquoise-- I agree that some shampoos claiming to straighten hair don't do that at all. And others that do, make hair kind of stiff and dull for some reason. So if you have a specific straightening shampoo in mind, you should definitely read reviews to see if it delivers what it claims.
The other thing is that sometimes shampoos take time to show their effects. The straightening shampoo I'm using right now didn't really do anything for my hair until I had gone through two-thirds of the bottle! It's straightening my hair quite well right now. If I hadn't continued using it for the sake of making it worth my money, I would have never known!