This is an interesting place to stay in San Telmo, especially if you are on a budget. It has the same price of a hostel with the convenience of not having to share the room with more people. But that's what it is, a private room. Not at all an “entire apartment“, as stated in the listing. It’s a room with en suite bathroom. If you want to use the kitchen appliances, you need to go downstairs to a common area (that’s shared with the real estate company that rents the place). It’s not that big of a deal, but it may throw some people off.
Other than that, location is great, near all good things the neighborhood has to offer; internet (5Mbps) works fine and I didn’t hear any street noise, not even on the weekend.
I’m using Airbnb as a substitute for my home office, so I spend many hours inside each place I rent. For that specific use, I don’t recommend it, as it can feel a little claustrophobic, and the mezzanine table/chairs are not very comfortable for working with a laptop. However, if you’re going to stay more time exploring Buenos Aires and later coming back to sleep, or if you’re on a budget and wants to pause the hostel life for a moment, by all means, go for it!