What can be said? She added charm and emotional depth to every episode she was in, (I think the only character she didn't *always* steal the scene from was Picard) and became a well loved character, despite being almost entirely superflous to the plot.
When I was a kid, I assumed that her backstory was shown at some point and if I just kept watching I'd get to find out what was up. I didn't really realize until I watched through the whole series as an adult that A) she isn't actually in that many episodes and B) you never find out what her story is. Whether intentionally or through a quirk of Whoopi Goldberg's acting schedule, Guinan preserves her mystery.