It's hard to reach a definite conclusion about Nikki as Rosalie at this point, since her biggest parts in the series are yet to come. She had a fairly small presence in Twilight, but I thought she did a nice job of getting the character established. The glare she gives Bella at the baseball game (when she's called out), crunching the salad bowl, etc., did a lot to convey Rosalie's antagonism towards Bella and her general attitude. In New Moon, she's barely there; Rosalie has two significant speeches in the book, which the script somewhat clumsily splices into one speech in the vote scene. I wasn't blown away by how Nikki handled that scene, but it seemed to me that Chris Weitz was giving the whole scene the rush treatment, so I give her a pass for that one. I'll be looking forward to seeing how she handles Rosalie's big moment in Eclipse (assuming that it makes it into the movie in recognizable form).
It's easy to say that Nikki doesn't measure up to the way Rosalie is described in the books. Rosalie is portrayed in the books as this absolutely breathtaking, unattainable beauty; I think it's fair to say that Nikki, while quite attractive, doesn't really have that unearthly quality. But does anyone else have it, either? If you consider actresses who are reasonably age-appropriate, and who would be realistic prospects for the role (e.g., forget about Scarlett Johannson), is there anyone who really has the kind of screen presence to live up to the image of Rosalie in the books? I've suggested that the only actress, ever, who could have really given a satisfactory Rosalie visually would have been Grace Kelly.
"May the hinges of friendship never rust, nor the wings of love lose a feather"--Scottish blessing