Just got back from doing it. One piece of advice I would add is if you cannot clear all border crossing stuff before noon stay in Pak Beng for the night. We tried to push through to OudomXai and we made it, but the last hour or so it was in darkness, cold and I had bugs everywhere. Plus you miss all the sights of a great road.
Another note the restaurant in Pak Beng where we stopped to have a late lunch told me the river is too low now for all the Mekong boat trips so you probably won't have to worry about getting a room.