I thought she was utterly brilliant in American Hustle. She was the wild card that kept everyone on their toes. Without her, the movie would have felt a bit too formula.
Golden Globe and Oscar voters agreed with you.
I like Rebecca Romijn for the same reason most of the rest of you do--as pretty as JLaw is, Rebecca Romijn has a prettier face and a better body. She fills the nude Mystique costume better than JLaw is capable of. JLaw matches Mystique from the comics better, however, in that she can deliver lines better than Romijn is capable of.
I was watching The Rogue Edition for the first time today and noticed what I like the best about JLaw in the role. It's her eyes. Even in those yellow contacts, her glance lingers on you (not a stare) after she decides on something and you think she's just gonna change her mind and kill everybody in the vincinity. It's a very playful glance (almost with a smile), fully aware of what she is capable of when your guard is down. Romijn (a sexpot in her own right) didn't have that, so her character always felt as predictable as that lady Terminator in T3. Plus I agree, her lines were too quick with no pay-off ("Because we shouldn't have to",) unless a generous co-star did the emotional heavy lifting to react to her.