We chose this hotel as our HQ during our 5-day city-trip in Rome. Main reason was the certainty that the NH family of hotels has to offer: modern, clean interior (even some design), central location, perfect for short stays.
NH Giustiniano is just that. It's located in a street that ends near the San Angelo fortress, and a couple of blocks next to the Vatican area. We've seen every Roman place of interest without any need for public transport of taxis: everything is really within walking distance (the Trevi fountain, the Spanish stairs, the Forum Romanum, the Vittoriano, the Colosseum, the Pantheon,...).
Just one hint: avoid its restaurant for evening diners. Portions are huge and lack taste (imagine a mozarrella/tomato dish without olive oil or pepper!). It's fine for breakfast, though. But at noon or in the evening try the San Marco pizzeria nearby.
Internet speed is competing with turtles, but who needs the Internet if you're in the center of Rome?