We stayed only one night at the Merulana Star on our way home from Naples but would happily have stayed longer. The place was nice and clean, and at least our room that had a window to the side street was perfectly quiet, even though the hotel is situated by a major street.
For the the little we payed for the night, breakfast or wifi wasn't included, but we prefer having our morning coffee at a cafe anyway.
The staff was very friendly, they gave us the map and showed how to best get to all the major sights from the hotel, so even if we had only one afternoon and night in Rome, we managed to see most of the "musts". No time was wasted in wondering about the logistics.
The hotel isn't marked very well, only a placate on the wall next to the door, so watch closely to the street numbers to find it. You have to buzz to get in, but the reception is always open.