We had a great stay at Carmen's. Central location near Plaza Alfalfa - great for local bars, shops and close to the sites.
The place was clean and comfy and the manager, Fatima, was very helpful. Nice big bathroom, though the hot water ran out very quickly.
The only real issue was that it was above a nightclub or bar that was loud and very busy with backpackers/students until the early hours every night. This might be a summer thing and didn't really affect us, but wouldn't be ideal for light sleepers.
We definitely stay here again.