As much as I liked the way this scene was done, it was kind of distracting to see how odd Billie looks now, and how enormously different from the way she was in series 1. I just couldn't find her convincing as her younger self, and that spoilt the scene for me. It was only 4/5 years ago after all, and most actresses would easily be able to pass convincingly for 5 years younger if the part demanded it.
But Billie has changed so much, (the teeth, losing masses of weight etc), she just looks like a different person now. It's hard to believe she's only 5 years older than the girl who looked so striking, fresh-faced and pretty in series 1, especially in Dalek and TEOTW.
Having said that, I'm very glad they did that scene the way they did. It was a nice idea to have Rose's "goodbye meeting" in the pre-Doctor past, before she fell in love with him. Much better than opening the can of worms that would have ensued if the present-day Rose had met the Doctor again. I guess RTD decided the Doctor clone was best left alone, too.