My girlfriend and I stayed at Luna di Marzo during August 2012, we found it to be a great hotel in a small, quiet town just outside Manarola. Even though the hotel is not in Riomaggiore itself, we were actually happier, as the little town of Volastra is very nice and quiet (bare in mind we could only find one restaurant (good by the way) in Volastra and the bus service up from Manarola stopped quite early, so if you wanted to go out late at night, this might not be the place for you).
While it wasn't in the centre of one of the big five towns, it was the perfect place to start the hike throughout Cinque Terre. Actually, I think the best walk was from the hotel to Corniglia, through the vineyards while still overlooking the sea, it was absolutely amazing. The main route from Manarola to Corniglia was closed during our stay anyway, so this was the perfect location for us. From Corniglia we were able to walk the coastal route through to Monterosso.
The hotel was clean, the room was large and the breakfast was great!
The service was good, i would definitely recommend this hotel for a couple..