Though we were here just one night, we very much enjoyed our stay at Hotel Arenal Green. The front-desk people were very friendly and helpful, the cabins are sweet. We found the rooms to be clean, and the shower was strong and hot. We very much enjoyed walking from the hotel up to the waterfall. We saw our first toucans and many other amazing birds along the way, and swimming in the waterfall was fun. All in all, this hotel is a great value!
The main reason I don't give the hotel a five-star rating is that the breakfast was only OK. All of the other places we stayed in Costa Rica had really outstanding breakfasts, but I would say this one was only so-so. Absolutely fine and entirely edible, but not amazing. Also, a very minor point, but I'd say the lighting is a little odd in the rooms -- for privacy you want to keep the curtains closed, and the electric lighting was either super-bright or too dim.
We went to bed very early, and the people in the cabin next to us were on their deck until 11pm or so. Even just regular talking (they weren't being loud) seemed fairly loud in our room as the cabins are very close to each other. But, because we went to bed so early, and they were not up late, we didn't bother asking them to quiet down. Again, for the value, this hotel can't be beat and I only describe our experience so future visitors aren't surprised.
Another point: Since we were there only a short time, it didn't matter to us, but in terms of location: It would be a long-ish walk into the town of La Fortuna from this hotel, so we did drive to town for lunch and dinner. That said, this hotel is within walking distance of the waterfall, so that's a big plus in its favor.
For the record, we never actually saw the volcano, due to the heavy fog that never lifted. But it's still a beautiful area, and we loved this little hotel, and found it to be an excellent value for the money paid. We (my husband, mother in law and I) all agreed that we would happily return to Hotel Arenal Green.