Ripley8 is one of the more intriguing aspects to A:R, in my opinion.
I thought Siggy did a good job of playing a woman who remembers being Ripley, but knows that she isn't.
Problem is (as with a lot of Whedon's writing) that a lot of the better moments are drowned in stupid puns and one-liners, which defuse the mood.
A:R will always be, to me, a good film wrapped up loosely within a bad film.