We had a wonderful experience at Arco Iris. The walk to the beach was easy and there was a grocery store, pharmacy and at least one bank w/ATM on the way.
Overall the room we got was fairly quiet and the air conditioning worked well. Nothing particularly special about the room itself but the bed was comfortable and it was cleaned daily. Oh, and at certain times of the day you could see and hear howler monkeys from the pool.
Breakfast was really good for a small mom & pop style (privately owned) hotel. It was freshly made to order and there were probably about eight options to choose from. I went with pancakes most days, my wife took the egg path on occasion, neither of us had any complaints.
The pool was pretty small but had well shaded options for seating/laying out when the heat and humidity is a bit much. There were never more than 2-4 other people there during our trip. Really, though, I imagine most people would walk the 7 minutes to the beach.
Sorry this review is a bit all over the place, but the general point is - it was worth it and we genuinely enjoyed our time while staying in Tamarindo.