The Space Apartment is a wonderful spot in an excellent location. It's a little tough to find (at first) but the area is perfect - blocks from Piazza Navona and within walking distance of St. Peter's and the Vatican. It's also surrounded by adorable shops and great little restaurants.
The apartment was large, we were only 2 but 3 (maybe even 4) people could easily stay there. Two bathrooms, one shower, the kitchen has a washer / dryer but be warned it takes 5 hours to do one load.
The apartment is really beautifully designed if you like contemporary homes. The windows open to the courtyard, and despite the heavily trafficked location, there was no noise at night.