I was in Hanoi last month and went to and from the airport in the $2 shuttle. As mentioned, it's fairly annoying because they wait for the whole shuttle to fill up (packed) before they will leave. Also, as with most things in Vietnam they charge foreigners more (I think it's 30,000 for the locals). The difference is small, but annoying. That said, it's cheap and means you don't have to deal with a taxi driver.

If you do take a taxi, make sure you are VERY clear about wanting to make a stop when you negotiate the price. And make sure the negotiation is for both of you. It's not unheard of to arrive at your destination and hear "it was $15 per person."

Viator has taxi services for about $40, as well. http://www.viator.com