Always book my airport transfer in Hanoi through this website: http://www.yourlocalbooking.com/

It's anywhere between half and 1/3 the price of a standard hotel shuttle. Cars were always brand new and drivers very pleasant. They'll be waiting in the arrivals lounge with your name on a piece of cardboard so you can't miss them either. Pick-up from the hotel was always on time, too.

I've been wanting to promote this site for a while now, glad finally a post came along where I could do so. Highly recommended ^