Considering the fact that the only way I can have an orgasm (apart from masturbation) is from oral sex, FUCK yes I would tell my partner if they sucked at it. My own interests are at stake!
However, I have only really had one partner in that sense, and from the First Fuck onwards, we've always made it a priority to be as open as possible with each other. I want to know if I am not satisfying him, and vice versa.
EDIT: Amended after reading Kurty's post... of course, I would not say, "You suck!"... (even if it were literally true)
But presenting things in a gentle way and making helpful suggestions, yes... both my partner and I do that. Unless we are doing a little domination action, then there's no gentleness.
