You cannot beat the location if you want to stay in the City Center. Most of the big tourist attraction are a 15 minute walk away. Castle St. Angelo and the Pantheon are about 10 minutes away and the spot where Caesar was murdered is about 5 minutes away.
There is a taxi stand in the Piazza across the street just to the north and bus stops there too. There are several great restaurants in the piazza just to the west of the apartment and a great little gelateria with good prices literally around the corner.
As far as the apartment goes, my advice would be to remember that this building is over 500 years old. It is not a modern hotel, so if that's what you're looking for, you should be looking at the Holiday Inn website. If you want a more fun, homespun feeling, this apartment is great.
We didn't spend a great deal of time in the apartment itself, but self check in was easy enough. The stairs were narrow and winding, like others have said, so bring smaller suitcases. I quite liked the staircase and the whole feel of the apartment. It was spacious! The pictures on the site made me think it was smaller than it actually turned out to be. It was fun to "live like a Roman" for a week (we live in the country, so city living was interesting). Everything we used in the apartment worked fine. The weather was perfect so we did not need the AC or heat, but the washer and refrigerator worked and the shower was hot.
If we travel back to Rome, I will use this apartment again.