I have stayed in a number of Airbnbs over the years and Monica's is an absolute gem. One learns to be a bit wary of photos that show a place that's too good to be true, but this apartment is 100% true to its images.
The flat is stylishly furnished and brimming with character - the mezzanine level and the huge skylights are unique and a clever use of the existing space, as well as making the flat very airy and light. The bed is long (bonus!) and very comfortable (there are sofa beds which I believe would fit another 2-3 people in the living room in relative comfort). There are two bathrooms, both with their own shower (the master bathroom has a huge walk-in which is particularly enjoyable). The kitchen is in a separate room with a closing door so the whole flat doesn't get filled with smoke and cooking odours, fully functional and has plenty of kitchenware to cook proper meals in. There is a large 4K flatscreen, and WiFi is fine for streaming services (preloaded on the Smart TV). We stayed in winter so can't comment on A/C for summer guests, but it is there. Like the heating it's controlled by the guests, not the building administrator. The skylights close if you want to sleep in.
Location is wonderful. Via Margutta itself is a famous artists' retreat and has been for centuries, with an elegant street with many boutiques and studios hiding a fascinating labyrinth of oddly shaped architectural nooks and crannies (within which the flat itself is located). You couldn't be more central but it's also very quiet, as the flat is well set back from the street. It comes with a charming little balcony which would be great in summer.
All the famous landmarks (Pantheon, Castel Sant'Angelo, Via del Corso, Villa Borghese etc) are within easy walking distance and there are shops for essentials and supermarkets within less than half a mile. If driving I would recommend street parking just north of piazza del popolo rather than getting sucked into the maze of the restricted access zone (ZTL) around the flat itself, which can be confusing to non-locals. You can park for free or on a pay-and-display street for around 6€ per day and still be within easy walking distance of the house.
Monica herself is a delight to deal with and extremely accommodating. This place really does go the extra mile.