I find Briony a fascinating and dynamic character. Ian McEwan has a talent for creating well-rounded, flawed, and appealing characters, and Briony is, in my opinion, one of his best. In the opening scenes, he paints her as an obsessively ordered child who craves attention and has a vivid imagination. He manages to convey all of those personality traits in just the first half of the opening sentence:
"The play-for which Briony had designed the posters, programs and tickets, constructed the sales booth out of folding screen tipped on its side, and lined the collection box in red crepe paper-was written by her in a two-day tempest of composition."
So, from the very start we get a glimpse of her intense personality. I personally find her a bit irritating; we have all known that child that likes to show off, and to have things just so, and it is usually hard to not be annoyed by them. They have a hard time just going with the flow, and accepting life for what it really is, and tend to get upset when they feel out of control. They are emotional and difficult to deal with, relate to, or enjoy. So, I find her a bit annoying, and I can relate to that annoyance, because it brings to mind people that I know that are like that.
Of course, when she falsely accuses Robbie, it is hard not to have harsh feelings against her. However, McEwan had done such a good job of painting her character by then, that it is entirely consistent with who she is. I couldn't imagine her NOT having done that. Of course I wish she hadn't, but it was in line with her personality.
Later, after a few years have passed and she has become fully aware of her awful mistake, I found myself feeling bad for her. What an awful predicament to be in! Her remores, and her work as a nurse helping dying soldiers has added an element of compassion to her, and she has loosened a lot of the rigid structure of her personality as a result. She is more likeable, not only because she realizes her mistake, but because she has learned from it, and is striving to change what made her make the mistake in the first place.
Those are my thoughts on Briony. She is a brilliantly crafted character, one that elicits strong emotions from the audience; if she can do that, then McEwan has succeeded in fleshing out a person that seems real enough to feel something for, whether good or bad. It will be interesting to hear other people's thoughts too!