You've been so good to me.
You'd been so good to me.
Please explain the different usage. And is it true that both examples imply that the person is no longer good to "me" now?
Well, "You've been so good to me", which is a contracted form of "You have been", means that, from an unknown (or unimportant or unspecific) point in the past until now, you have treated the speaker well. If it were "You were", it would be a known (or important or specific) time in the past whereas "You have been" is an undisclosed time in the past up until the present. For example:
"You have been so good to me these past three months. I am so grateful for it."
"You were so good to me three months ago when we went to Seattle."
As for the "You'd been" in your example, which is a contracted form of "You had been", the speaker is telling you that, starting from an unknown (or unimportant or unspecific) time in the past up until some point later in the past, you had treated the speaker well, but something has changed and you are apparently no longer treating the person well or, at least, as well as you had been prior to some event. For example:
"You had been so good to me these past three months, but something has changed. You're now completely ignoring me and, when you do speak, you're very rude to me."
I hope that might have helped you out. Take care and good luck!