The street is very small and plain. I know it very well. You will be between lovely Via Giulia (and, in fact, around the block from Raphael's old house and Chiesa Fiorentini, where the great Borromini is buried--he used to live here too) and the busy, but very handy Corso Vittorio II. I love the location because you are near Navona, Campo dei Fiori, the Vatican etc., and, with time, can walk everywhere. When you want to take a bus it's very easy; just walk up to the Corso Vittorio and catch one straight down to the Piazza Venezia to get to the Forums, the Trevi and more.
It will be silent at night, but you have a few nice restaurants nearby, you are around the corner from a wonderful coffee shop (two actually, one on Via Giulia and one on the Corso Vittorio; the former are artists, making cappuccino with hearts on top, the other is excellent, Roman coffee, no nonsense). You will also have a large grocery store just next to the coffee shop on Corso, a charming family-run cut pizza and bakery place on via Banchi Vecchi, and the nicest set of characters a neighborhood can have.
I can not speak for the apartment, but I can say that RomePower have been in business a long time and are very reputable.