If I could, I would give Arenal Springs 4.5 stars. There's a lot to like here, and the misses are pretty minor. First, the grounds are just beautiful. This place really fits into the landscape, and the views of the volcano are great.
The rooms are also very comfortable. Not fancy really, but the beds are firm and comfortable, the air conditioning works great, and there's plenty of hot water. There's a mini fridge, a safe and plenty of shelves/hangers.
The toiletries could be improved, and I'm surprised for a place that touts being eco-friendly that they would be using tiny plastic bottles for shampoo and conditioner. Also, the bathrooms could use some re-caulking as there were many miniscule bugs. Also, maybe some weatherstripping by the door would help for the same reason. But not a big deal.
The hot springs are perfect. A number of pools, various temperatures, two swim-up bars. Perhaps some additional chairs or umbrellas would help, but there are not that many people at the pools for the most part. You won't have trouble finding chairs, but you may have to look around a bit.
We had a car (I absolutely recommend one), so we usually only had breakfast at the hotel. It was fine, nothing special. But they have an omelette station, a sandwich press for hot sandwiches, fresh fruit from the area (bananas, pineapple, watermelon, papaya) and some other hot and cold items.
And overall, the staff is fantastic. Everyone was very friendly and accomodating.