We were a group of 5 adults and stayed on Merlijn's houseboat for three nights. It was a great experience and we enjoyed it very much. We stayed there in December when it was pretty cold, and we were slightly worried that the cold river might cool the boat quite significantly. However, the heating was totally great and we did not have to freeze at all.
The bathroom was fairly basic and probably not for the posh or spoilt types, but is was ok for us. It just takes a bit of organisation to get / keep everything dry after a shower.
The only criticism would be that the guest room bed was really not suitable for an adult, because the distance between the walls was really to narrow so one would be lying in bed with the feet "standing" on the wall. Also, the floor on which the mattress lied was not flat, so we really couldn't sleep deeply in that bed, and we resorted to moving the mattress to the living room to find a flat stretch of floor. Ultimately, the guest room should not be advertised as an adult room after all, I guess.
Overall it was a great experience, the boat was located at a convenient spot and Merlijn was a great host, who was very very friendly with all of his communications. It was a real pleasure to have stayed on his boat and I would recommend it to anyone who doesn't mind be a little bit less comfort than a typical hotel room.