You'll get opinions on both sides. It's widely believed to be very dangerous, but that's usually spoken by people who've never even done it. The two times of year I would hesitate are peak dry and peak wet season (aka "rock" season and "log" season), but now we're right in between so the chances of bumping into anything large and solid are relatively low.
I've taken speed boats on the Mekong several times and never found it scary or overly dangerous, but it is a bit uncomfortable after a few hours. They normally pack in six people and their bags get lashed to the bow so if you can manage to sit in front and rest your legs on the bags it's more comfortable. Oh and one common sense item: sunscreen. When you're moving that fast it's easy not to notice yourself burning.