We had a very pleasant stay at Nakakiri. We stayed in a Pool View room, which was spacious. Decor was nothing special and the room lacked any real character but it t=did the job and had a huge bathroom.
We weer 2 adults and 2 children and there was plenty of space. It was nice to be near the pool and worth the extra. The standard rooms re not well located and although the rafthouse looked interesting it was a long walk down to the river.
Basically the resort is fine. Standards are OK, service is OK, food is quite good, but nothing special at all.
Plus it is set up as a conference and group tour resort and bus loads of locals on a company jolly or buses of Russian tourists dramatically change the atmosphere and noise levels. We had both while we were there and although they did not spoil the stay they did change the experience.
The big plus is the location as there are lots of places to visit in the vicinity (hot springs, waterfalls, Sangkhlabuuri etc..) and this is a really beautiful part of Thailand.
In short, don't expect a small, local resort. This is a quite a big, busy resort. Not at all a bad one, but not a great one either but it is the best in the area and the area is worth visiting so if in doubt.... GO TRY IT!