Really enjoyed our stay at Casa Luna. The room was clean, spacious with the best bathroom we had during our trip and the comfiest bed. Try and get an upstairs room as the views of the Arenal Volcano will be better from the balcony. Nice pool. Breakfast was buffet style and OK but not as good as in other places in Costa Rica, and the restaurant was quite expensive with limited veggie options. The staff were helpful, if a little clinical at times.
I picked this place as I thought it would be walking distance from town - it is, but it's a good 30-40 min walk and I wouldn't do it at night. The Hotel is however within walking distance of the La Fortuna Waterfall which we really enjoyed (a pleasant walk past farms and then LOTS of steps down into the valley, but great to swim in the jungle!).