I stayed at Luna Llena for 5 nights in the dorm room. It's a perfectly nice hotel - up the hill from the main action (which means you can avoid feeling guilty about lying on the beach all day - it's a good workout!), with some nice views of the ocean and a good-sized lounge area. There is one dorm room with 5 beds, which - at $10 per night - makes a great deal. The room is perfectly comfortable, complete with fans and lock boxes, but it is right next to the kitchen and lounge area, so it can be noisy. The hotel has a strict 10 pm quiet policy, but staff begins cleaning and making coffee *bright* and early which will disturb a light sleeper like myself. The guest at the hotel were very friendly, but because most of the rooms are private, Luna Llena lacks that true "hostel" feel, if that's what you're looking for.
I found the staff very helpful in terms of planning activities and transportation. They will offer lots of recommendations in terms of places to stay elsewhere in Costa Rica -- just keep in mind that, as always, they are usually based on a mutual agreement/commission policy.
A few things that would have kicked my experience at the hotel up a notch: breakfast (at least fruit and/or bread), hammocks (a staple for any beach getaway), and more monkeys! (Despite my near-constant vigil, I didn't see any from Luna Llena; when I went to visit friends at the hotel next door, they were everywhere!)