Does keratin shampoo actually work or is it a scam?

Don't expect a keratin shampoo to restore your hair's structure from the inside - it's impossible because keratin molecules are too large and can't penetrate deeper than the outer layer. That said, these shampoos do work as a conditioner: your hair becomes smoother, shinier, and more manageable because the keratin settles on the surface and seals down the cuticles. There's an effect, but it's temporary - it washes out with every wash.