Basically, this apartment in the middle of Kolonaki has everything to offer that one needs, but unfortunately there where problems. A couple of hours before my arrival, I received a message to be careful with the fuse (because the apartment can't handle too much use of electricity all at once) and turn on the fuse for the boiler only when to use warm water. It also stated that I had to turn the fuse on first, then open the tap and wait at least for 15 minutes until warm water would be there.
Though I didn't read the manual carefully and after waiting for 20 minutes, I asked Evan (he isn't the owner of the apartment) what was wrong. He guided me trough it, but still there was no warm water. So no shower for me (winter in Athens can still be quite cold). The next day I tried again and waited for over one hour. Still no warm water. Evan did his best and a technician came over to fix the problem. On the my third night in Athens I finally could take a hot shower.
After thanking Evan for everything and after checking out, I asked for a discount from the owner. He or she agreed to give me a 30% discount on a total of 297 euros, but until now only 30 euros has been paid back. Due to the hassle we had earlier that week, I decided to leave things as they were.
Besides all the trouble with the fuse and boiler, I can still recommend this apartment. It is in the center of Kolonaki and basically has everything to offer one needs. Important to know: the boiler is fixed but the fuse problem was not. I hope the owner will do his or her best to repair this too and I wish Evan all the best.