Don't even see why this is an issue, unless you are exclusively attracted to people who remind you of your ex, if that's the case then that person has issues or needs to break the mold fast...
Only seen her in The Town and Vicki Barcelona but just from seeing her in those 2 films, I can safely say I'd marry her immediately. She just seems so down to planet, kind, cool and passionate. I've never heard her speak British and that may make me like her less or it may make me like her even more. I prospect I'm unwilling to engage in.
But on topic, she doesn't look like anybody I've dated. But for those that do, I just imagine the sex may be the same, eg Jessica Alba is a damned log. You can't teach passion.