If your goal is ease of walking around the Plaka neighborhood, Syntagma Square/Parliament, and the Acropolis, Nefeli is an excellent choice. Great central location near everything you want but just off the tourist streets, in a quieter neighborhood. (Tricky access by car, however!)
Rooms were basic and comfortable. Price was great, considering the fantastic location. Other hotels in the neighborhood are all $40+ more expensive. There is a parking garage a few buildings away; street parking near the hotel is unlikely given the tight, old city streets. For travelers with no car, this hotel is ideal.
Desk clerks were mainly helpful, some extremely so! Breakfast was a bit disappointing, given the huge spread most Greek hotels offer, but adequate. I would go back on future trips to Athens.