Valerio's location cannot be beat. That was without a doubt the best part about his place. You walk out and you are in arguably the coolest neighborhood in Rome (Trastevere). If you are in your 20's or 30's or just like to go out to eat, drink, and enjoy the people, this is the spot for you.
I have seen people complain about the stairs. This is nothing compared to places like Amalfi, Capri, or other high places in Italy. Burn off some of those carbonara carbs! It is only like 50 steps.
The apartment is quite small. My wife and I couldn't even really open our suitcases comfortably without blocking the fridge or the walkway go our room. It is doable for a couple, but perfect for a single. Definitely not made for more than two people.