Backstory works best for me when it applies to the main story, when it seems to come up naturally. I find backstory less obtrusive in the middle of the main story. At the beginning, the writer is more obviously filling in the blanks. The exception, of course, is if the second book picks up right where the first left off, but I haven't read many tightly continuous series. I prefer the kind I can pick up at any point or get back into without much fuss.
I do cut writers some slack using the same information/description in multiple books. I understand that they have to court new readers and keep returning readers engaged. And after all, if I haven't read about the series protagonist in a year or more, it helps to have a reminder how she looks, how she sees herself, and where she is in life.