My wife and 4yo daughter booked a taxi through a café we had lunch at near St Joseph cathedral. The Mai Linh taxi arrived and we advised we wanted to go to the Ho Chi Minh Museum (famous and well known landmark) and showed the driver the local map and pointed to it. This is about a 20 minute walk but it was hot and hard for my 4yo.
Driver seemed not to know how to get to the Museum (??) and went around it on a different road. I could see where we were on the map and had to guide him back. He would not turn off the meter and kept pulling over and not driving. Eventually we just got out not knowing exactly where we were which is a little scary with a young one. Cost was $142,000 D (about $70,000 too much) and we recorded the taxi number.
Our hotel advised that they attempted to report this to Mai Linh with no response. I would not trust Mai Linh taxi's and the company do not seem to respond to these issues. I would advise if you catch a taxi you should al least know roughly where you are going and what the rough cost should be. I could have directed him myself it was that close. Also take a photo of the taxi before getting in and the driver which we did later to attempt to keep them honest and to have evidence if something happens.
Hanoi is a great city and it is a shame that the taxi's can not be trusted. It would help tourism a lot if they policed this better.