When you get off the boat at Pak Beng, you are surrounded by people trying to get you in their hotel. I went with these guys on the proviso that i was not required to stay, but only wanted a look.
The rooms were super cheap, and clean, bathroom was ok, there were fans (no air-con as Pak Beng apparently runs off generators, so I am unsure if any accommodation has air-con). Wifi was included, breakfast was not, but you could buy it in their restaurant. Fantastic views from rooms and restaurant over the Mekong.
Considering you HAVE to stay in Pak Beng for the boat trip, I highly recommend this place.