The bottom line is if you can afford to fly to Halong Bay with Hai Au (They are only company to fly there) you should do it.
I was with my family of five (kids 11, 8,8) and it was an amazing experience. I've never done the bus trip, but this must be better than a 3-4 hour bus trip each way. Note that we booked a last minute fare so we did not pay the normal price.
Also, for full disclosure, I ended up randomly meeting one of the Canadian pilots, Justin (I needed a barber recommendation) on the streets of Tay Ho on our first morning in Hanoi and he ended up showing my family around while we were there. This was before we booked our flight, but we had seen ads for the service and were planning on booking it. Small world.
Anyway-Justin wasn't flying the day we went so we flew with Dave-also from Canada.
Here is the lowdown:
Canadian pilots with Vietnamese co-pilots. I'm no aviator, but my sense is that if you can fly a float plane in Canada, flying in Vietnam probably isn't as challenging. You check in at the domestic airport. We didn't pay when we booked online-they took payment at the airport check in which I thought was kinda strange. Then their credit card machine didn't work and I had to pay half the fare in cash (basically I had to take the maximum daily amount at the ATM). You take a bus on the tarmac to the plane. They use brand new Cessnas- Justin said it was like flying a Cadillac. It was just my family of 5 and one other person in a plane that seats 12 or 15 people I think. FYI no bathroom on the plane, but it's a really short flight anyway. If you are not used to flying in a smaller plane you might think it's a little loud, so I suggested they provided headsets or ear plugs. I didn't think it was a big deal, but you know some people- I am trying to nit pick here for the sake of objectivity.
You take a more scenic route on the way to Halong Bay and it is just amazing. If you are on a boat it's hard to get the scale of it all. You cruise over the bay for about 15 minutes and then land on the water and taxi up a boat ramp as the tourists on the ferry nearby take pictures of this really cool plane that just switched from water to land. As you depart you are met by a 15 seat golf cart with cold towels and water and wait in their brand new lounge (with coffee/sodas/water) and then they take you to whatever boat you booked.
On the way back it's more of a direct flight. It was just my family and another couple on that flight. We did encounter some rain and lightning and it was a little bumpy so keep that in mind. I've been on small planes before and it didn't bother me. My kids didn't even notice. My wife was a little more sensitive, so it bothered her more. They mentioned that sometimes they have to cancel the flight due to weather in which case they would arrange for the traditional bus so you don't miss your boat-they all usually leave around1200/100pm.
For me it was totally worth the splurge. When are you ever going to fly in the float plane above one of the most beautiful places in the world (in my opinion)? Plus you are not going to be as tired on the ride to and from. From what I understand it's not the biggest highway to Halong Bay and you could hit traffic. I got to know the pilots personally (even said hi to Dave as he was biking down the street one day) so I might be a little biased. But even so I'm not going to compromise when it comes down to my family's safety.
In my opinion its the only way to go. Hope to see you guys again soon!