La Casina is a brilliant base to see all the sites. Its in walking distance of The Pantheon, the Spanish Steps etc, 2 mins from Piazza Navona, with loads of restaurants on the doorstep. Buses can be caught down the riverbank or beside piazza Navonna.
The flat is tiny, but has everything you need, and the sofa / bed is really comfy as either. Also Fabio, who owns it, is really friendly and helpful.
So five stars from us - we liked staying there and would recommend it to anyone.