(In case you haven't read the series, you may find some of this a squidge spoiler-y. I don't think it is, but fair warning.)
In the books, I understand Ygritte’s purpose as a means of character development for Jon Snow, however I feel as though her character’s insertion could have been better integrated into the story-line (or at least done differently). She quickly turned into a character that only seemed to exist to get Jon’s rocks off. By the time her part of the books was over I felt as though she hadn’t actually been important to the story line at all – sure she more or less keeps him alive, but I feel like a lot of things surrounding her were fluff. Even to the present point, three books later, it feels like GRRM is trying to force some form of influence from her interactions with Snow.
(To be fair, I feel that after the first book, only the third’s ending has been worth reading. And the only reason I’m still reading the books is because I care about what happens to Tyrion.)
So far, she's not annoying me in the series. But I think that's because we've got so little of her. I'm not looking forward to Season 3's portion in the North.
And I found it extremely irritating that a good 50% of her dialog in the book seemed to be her catch phrase of "You know nothing, Jon Snow."