Oh yeah, I've been there!
I always take the position that it's not my song, so if the musician is not satisfied, they'd know better. Although it is easy for a musician to lose perspective after too many takes (especially when marijuana or alcohol is involved!). Sometimes if it's not happening I'll suggest a five minute break, or to move on and come back to it.
Besides, if you're getting paid by the hour like alot studios and engineers, what's the complaint?
