I had too - until recently. I actually found a flight with Vietnam Airlines that was 15% cheaper when I booked it through booking.com - much to my surprise.
That being said, for the same upcoming trip to Vietnam, my six-week stay at a hotel, if booked through booking.com, would have cost just over $2000. I then asked a friend in Vietnam to go directly to the hotel for me and ask the price: Less than $1,300! And no advance booking required, as the hotel said they were nearly empty, as it's their low season. So my strategy going forward for longer stays in places where I don't have a friend who can go to the hotel to ask the price for me, is to just book the first night or two at any decent hotel, then to walk around and try to negotiate a deal for a longer stay.