We spent four nights in this apartment in late January and we enjoyed it very much. It is very safe and secure, as noted by others. The location in San Telmo is great, with access to endless restaurants and cafes within a few minutes walk. The Sunday street fair (not to be missed!) is a five minute walk.
A negative review was posted in early January, shortly before we started our rental, and it made us hesitate. The review turns out to be peculiarly grumpy and at least partially inaccurate. The response from the owner was detailed and addressed each point appropriately. Additionally, I am pleased to see that another favorable review has been posted since then. A few points in the negative review are worth commenting on. First, I can't say for sure how clean the apartment was for that rental, but for ours it was as clean as one could possibly wish. Ana and Ivo were very helpful and it was nice to know that they were available for anything we might need (we didn't). I agree that the kitchen could be better stocked, and Ana seems aware of that. The negative reviewer wrote "It was difficult to be totally comfortable there." Well, we had no trouble being comfortable. Temperatures got up to 92 F during our stay, but with the two bedroom air conditioners running the apartment was comfortable (would have been more comfortable with more air conditioning, but not bad at all). You should be aware that there are stairs to deal with within the apartment: to the kitchen, within the downstairs bathroom, and to the upstairs bedroom and bath. If you have mobility or balance problems, this apartment is not for you. But those are not major points otherwise.
All in all, we found this to be a pleasant and comfortable apartment at a reasonable price, and we would stay there again if we return to Buenos Aires.