I stayed 3 nights here rather than in town because I was arriving and leaving from the airport and the price was right. I liked it and would definitely return. These were the highlights for me:
Room: As a titanium I got an executive room on the lounge level (3rd floor) that was quiet and had no noise from the planes landing/taking off nearby. I have had very bad experiences at some airport Marriotts with aircraft noise (DFW).
Lounge: I actually preferred breakfast here because it was smaller, had better fruit than I found in the restaurant (incl. mango and papaya) and was more relaxed. I appreciated that starting early afternoon the lounge had a full buffet of cold foods + cava/sangria + desserts. The "hot food" in the evening was not impressive, more the frozen breaded things (empanadas, breaded calamari and fish, all lacking flavor) that most US Marriotts have gravitated towards, rather than real hot dishes you can still find at many European Marriotts.
Pool and gym: Pool open 1100-2100 and was nice, though crowded at times. Gym was 24 hours, quite big with good array of equipment and never seemed crowded. Towels (full sized and small) were there for the taking and that was convenient the one day I missed room service due to working in the room.
Bus into town: I went into town sightseeing 2 days and found it easy to take the bus from the hotel (buses come every 5 mins, cost 2 Euros (cash only, they don't take anything bigger than 5 Euro note) and it seems they all go to Avenida de America bus station, which connects with several metro lines that can take you anywhere in Madrid). You need a separate ticket for the metro and buses in Madrid Zone A; I bought a transport card at Avenida de America for 2.5 Euros and loaded it with 10 bus rides for only 6.5 Euros (apparently 50% off until end 2024) and each of the 10 rides was good for 60 minutes with transfers (within Zone A). To return to the hotel, I returned to Avenida de America, got in line for the next interurbano (one level up from the metro, not 2 levels up where the long distance buses are located) heading to the Marriott Auditorium as indicated by Google Maps, told the bus driver I was going to the Marriott, paid 2 Euros, and rode 5 stops to the hotel. Madrid has an impressive transport system that is easy to decipher and makes getting around town easy.
Airport shuttle: I had no problem taking it in both directions. Both directions were crowded, so leaving the hotel it's wise to get down about 10 minutes before departure because there is a line that forms in the last 5 minutes or so before departure. Frankly, it was more the luggage than the number of people that filled it up. I was surprised that some groups with 2 enormous suitcases per person didn't just take a taxi or Uber as it would have been much easier on them.