This place has a wonderful vantage of the volcano, excellent service, a vast selection of soft drinks that will knock your socks off, and decent pricing. I am not sure you can do better in the whole of the Arenal region.
I recommend the agua dulces and the milkshakes and the cocoa. The Volcano Rice dish is also wonderful (with chicken, is my preference).
The restaurant is also dog-friendly, and the staff is very polite.
Go. Eat there. As soon as you can.