We really loved our stay at the Blue Apartment. The bedroom is huge and nicely decorated, the small kitchen is well-equipped if you wish to use it (but with tons of excellent Trastevere restaurants at your doorstep - why would you?), and Giuseppe was a great host and took care of everything we needed. The apartment is within 10 minutes walk to central sites such as Campo di Fiori and Piazza Navona, and the bus stop for everything farther is a 10-minute stroll away.
The only downside you should be aware of (and the only reason why I didn't rate this place with 5 stars) is the shower - the local water heater only supplied hot water for 8-10 minutes at best, so two people cannot take a shower one after the other, but instead need to wait for quite a while for the water to reheat. For us this was an issue - others may find it less troubling.