Part of why she's so cool is, aside from the very last scene, she doesn't make any stupid-movie-character mistakes: for example, when the good guys are shooting at her, she takes cover behind her army of robots.
Also, she uses her army of robots.
Because this is a movie about fake memories and deception, quite a few characters have alternate identities. Beckinsale changes her attitude and demeanor so much that it's almost like seeing two different actors:
For Farrell all they could think to do was give him a goatee, since he either only knows one facial expression or someone off camera is violently electrocuting him every time he lowers his eyebrows.
Beckinsale carries so much menace in each fight scene that she single-handedly does the job that the writers failed at: she's so cool she'll make you care what happens next. The problem here is that her best scenes aren't available on YouTube, and they don't really hold up when I type them out. Like this one:
BECKINSALE punches SOME GUY in the NECK
BECKINSALE: When you get your breath back, you're going to tell me everything that man said to you, starting from the beginning.
SOME GUY: Who are you?
BECKINSALE: I'm his wife.
Idiotic, right? But it's so goddamn cool the way she does it, and I'm not even sure why, because I'm not very smart or even qualified to have this job writing jokes for you fine people. My point is that Beckinsale injects cocky menace into a boring, cookie-cutter character, and it's allllllllllllmost but not quite worth watching the movie for.