i wrote my very first blog entry on this topic ... i simply adore being told those two words by my (Almost) Master - being told them in any other context is an entirely different matter, mind you - and miners_girl, 'glowing' is a great way to describe a response.
i think that part of it for me is that it takes me into a much less complicated place than my adult/"real" world life generally is. When i please my (Almost) Master, He tells me i am good. And it's that simple.
But yes, i 100% agree with those who are saying that if you don't like anything your Dom calls you intending affection, then let Him know outside of the play session. Just like negotiating lists for limits, things you want to try, things He won't do, etc etc ... words and phrases and language is so key to keeping things flowing easily when playing. i know that for me, His "good girl" and hair stroking lets me take much more from His crop or flogger than i would otherwise be able to endure. And that wouldn't be the case if the words didn't melt me so.