Although my stay at Moreno 550 started off rocky, it was ultimately a good experience, and I would recommend it to friends, and when I come back to BA, I would stay here again.
This is from an American perspective, for context and standardizing.
The small nits first. The front desk security only speaks Spanish - so bring your translator app. Check-in was not super smooth, as there were a few moments of confusion, but ultimately I got in. Once you're in the system, security to the building is very good, and everything was smooth.
The doors here have electronic locks. I had a hard time getting in initially, and later during the stay. Just a heads-up, the lock design is suspect, and the batteries don't always make good contact, or the batteries are dead. Security knows this, and can fix it. So don't panic.
The bed is super firm. Although there's a terrace, most units are facing inner courtyards, so you stare at another unit - so really no view. There's not enough chairs to store your stuff, or a luggage rack. Make sure to bring a power adapter - unlike other places, they don't provide one.
All small nits though. Ultimately, it's clean (super important to me, they actually wipe the floors), modern, and spacious. And the location is great. You've got a Dia and Carrefour close by. Subte stations are within minutes walk. San Telmo is a quick walk. The area is safe, and quiet. The staff is reachable by email, and responsive. Good value.
Recommended, and I will repeat.