I had a really pleasant stay at Oltians apartment and would recommend it to all.
The room is small but it has everything you need - the bad is comfy, the kitchen is adequatly equipped and all that.
The bathroom was also small but was enough for me - good shower, hot water, etc
The check in and check out are very easy - you just follow the directions sent by Oltian and no need for any in person interaction.
The location is very close to the center and main attractions - a 5-10 min walk from all that.
The roof is has a very cool view you can go up there and have a couple of beers and chill looking at the acropolis view.
I saw some people complaining on how the neighbourhhod is dangerous - well first of all the apartment is on the other side of the street of a police station. And secound - Athens might have a problem with homelesness and drug use on the streets and all of the neighbourhoods close to the city center look the same - so if you want to live in a walking distance to the city center you will see that since this is how the city looks. I personally didn't feel threatened or that it was dangerous and I was certainly moving around day and night around there. If it calms your nerves I think Oltian aparment location is one of the better situated since the police station is right across the street.