Fantastic location! It's quite inside the apartment, but as soon as you open the apartment door (still inside the building), you can hear the Trevi Fountain and tourist bustle - it's a pretty special sound and reminds me of the language audio lessons you get as a child!
Unfortunately we found the bed very uncomfortable, and the room is a bit more shabby than the photos suggest. The amenities are minimal so bring your own shampoo and shower gel etc. There is at least hand soap and towels. It's also not particularly suitable for cooking - that's possible but I wouldn't bank on it.
This place is all about the location. As long as you don't expect more than that, it's a great place to stay. Hopefully the host will get a new mattress and then all will be well.