If it is a special occasion, I wouldn't go there. I've been twice (once before and once after a holiday in Vietnam, both times within the last year), and it was awful both times. The food is just not good, and not very Vietnamese either! I thought it was just the choices that i made, but the people I was out with thought it was awful too...
That said, it is always busy, so perhaps it is just us - I wouldn't want you to be disappointed on a special occasion though!