Leaving four stars because although the apartment was nice and well located, the check-in process was a major inconvenience. The earliest check-in time listed is 14.00 but the local host Stefano informed us that the earliest he could do is 15.00. When my brother and I entered the apartment, the cleaning staff was still in the middle of cleaning the place from previous guests, and we had to wait until 16.40 for them to finish - almost 2h later than the agreed check-in time. When we raised the issue of a missing second blanket for the sofa bed (after having requested it twice via email), there was a lot of finger pointing by the host, although it was eventually resolved.
The floorboards in the middle of the living room are pointing up at a sharp angle which can be dangerous and could injure someone's foot if they're not careful, so I hope they fix this soon. The apartment is advertised with a washing machine but it did not have detergent so we had to buy some. The bathroom gets very humid and should have a mechanical vent installed.
Other than the above inconveniences, we enjoyed staying there for three nights. It is located on a vibrant street in Trastevere which could be a problem for some people in terms of street noise, but we liked being so centrally located and were able to sleep through the night. I also liked the unconventional, funky loft apartment design, although having the only bathroom on the second floor is not ideal if you also have someone staying downstairs on the sofa bed. So overall, five stars for location and design, three for check-in hassles/service, averages out to a four-star review.