My friend and I stayed in Nicolas's dreamy apartment for a little over a month at the tail end of our year-long round-the-world trip. It was magical. The apartment is just how it seems in the photos - grand, elegant, thoughtfully designed, and incredibly comfortable. The art - from the walls, to the CD drawer - was engaging and impressive. We work remotely, and the wifi connection and little spaces throughout the flat made it easy to focus and be productive when we needed to. The kitchen was robust enough to cook soups and roasts from time to time. The beds are comfortable, and the rooms are big (though you should know they share a wall). The building is quiet, friendly, and safe. We had everything we needed to feel like we lived there for the month, so shorter stays would find more than enough amenities.
The area is interesting - central and historic - which is a blessing and a curse, because it can also be a bit sketchy at night, and is directly along the people's protest route; making transport inconvenient at times, but also giving you a great view of the life and passion of the people in the city. The building is directly above a metro stop (which also serves as an informal taxi stand), and along the route of all the key buses, so traveling around town is incredibly easy.
Check in went smoothly and Nicolas or his sister were available if we needed anything at all. Although he was traveling for most of the month, it was such a joy to meet Nicolas and his partner at the end of our visit - they're just as pleasant, friendly, and charming as you'd expect.
If you have the chance to rent this place out, don't miss it! I can't think of a better home base for a stay in Buenos Aires.