My boyfriend and I stayed here for one night after the Gibbon Experience. We stayed in a double aircon room in 'zone A'. We stayed last week (July) and there was no need to pre book in Huay Xai. All guest houses we visited had spare rooms so you could see the room before committing.
We paid 140,500kip for our double room with bathroom (flushing toilet), aircon, tv and a fridge. Bear in mind that this is primarily a border town and this guesthouse is one of the cheaper ones we found. Having realistic expectations, we were impressed with this place. It was a basic, clean room that provided a decent shower and a good night's sleep.
The man on reception spoke good English and helped us book our onward slow boat tickets.
The included breakfast makes this place really good value for Huay Xai - it is in the attached restaurant next door. The only real issue we had is the wifi was poor, but I could pick up the attached restaurant's wifi through the wall.