The wife and I have done it several times, too. It is one of our favorite rides. We usually stay overnight in Na Haeo and then ride through along the Mekong.
We never leave the river, even when the road signs tell you to turn right. There is always a concrete road along the river and those old roads are so much more beautiful for scenery.
Watch out for huge potholes though and lots of gravel, but overall, it's pretty safe, if you adjust your speed to the conditions.
Kurt
The more people I meet, the better I like my dog.