The house was spacious and had everything you could possibly need. Initially I thought our location was in a bit run down area, but there was a whole area of nice restaurants and cafes a block down from where we were located, much quieter and less touristy than monastiraki, ermou and surrounds. Yes there are some streets I would avoid (as there are in most of central Athens), but this one generally had a bit of foot traffic and if you follow Stelios' advice you should be right. All the tourist areas were pretty close, monastiraki metro station is a short 4-5 minute stroll and is on 2 of the 3 the metro lines, or one stop from syntagma to get you to the third.
As to the title modern and comfy flat, that's exactly was it is. Beds were comfy, TV was great, washing machine did the work, all pretty good. My wife, who is picky as, said it was the best accommodation during our five week Greece trip. She also wanted me to highlight in the review that Stelios place was the only accommodation during this trip to provide washing powder with the washing machine so we didn't have to go out and unnecessarily buy a whole box. In addition to these little touches Stelios provided ear plugs, which had us a bit worried, but after the first night we got used to the general hubbub. Even though we were there for a week, we don't believe the restaurant below opened so I can't comment on it (unless it was really quiet).
Overall Stelios tried to make sure we enjoyed ourselves. Even though his place is pretty popular, he also arranged for us to leave the room a bit later and offered to store our bags for our very late flight.