Summary: Great apartment, well located for transport and remote work (fast internet).
Detailed review:
Terrific apartment conveniently located to the Atocha train station and metro. Artsy, somewhat eccentric design and decorations gave it character.
Remote work: Table to work, fast internet (I tested 300M+ on my iPad Air, faster than I have at home). Seemed to be reliable also makes this perfect for Digital Nomads or anyone needing to do some work while on vacation.
Patio was bigger/better/more quiet than I expected. A bit of an oasis in busy Madrid. Couch was tested for siesta and passed.
~5 min walk to Atocha station, perfect for using as a base for day trips outside of Madrid. Atocha metro a few mins away giving you access to the rest of Madrid.
Good pizza place right nearby, small market to buy drinks/beer from slightly grumpy owner also close.
Several cafes right outside also but we didn't try any of them.
Washer worked fine. Dryer I didn't use, line dried instead.
Would stay again for sure.
Only general comment - first time in Madrid for me. Atocha is not the "center of night life". If you want that you should look to stay closer to Puerta de Sol or Plaza Mayor. You probably won't get a patio there but maybe a terrace of balcony. But Alvaro's place is only 3 metro stops from Puerta de Sol. Depends on what you prefer.
Alvaro was very responsive to questions and follow-up, looking forward to staying in Madrid again at his place.