The one time I worked with McKinsey, they were "OK." The partner and one of the two associates were sharp. (The other mostly just arrogant.) On the one hand, they didn't come to any conclusion markedly different from what we (the product people) had already concluded.

That said, I'm sure they gave upper management some validation that we weren't just drinking our own Koolaid. And they gave business planning staff a fancy spreadsheet model to play with--which helped keep said staff out of our hair.

Ultimately, the product was fairly successful although the company couldn't really overcome a bad strategic position.