Staying in a hotel in Omonia is a bit like staying in a hotel in Brixton - you just need it to be on a nice street close to the metro. The Achillion is lovely, the staff are very helpful and accommodating, and late check-in (I arrived after midnight) was not a problem. It is opposite a pharmacy and a café (K29), there's 2 kiosks and multiple coffee shops to the left, and the metro station.
I was using it as a base for business/family visit, so I had a lot to do in Athens and it was ideal for me in terms of price, location, value for money etc. WiFi was patchy but perfectly serviceable for my purposes and I was on the 7th floor in a back corner room.
The hotel serves breakfast, lunch and dinner but be aware that there are time slots for different floors. Breakfast is served from 7am-10am but there are 7 floors in the hotel and a small restaurant, so Floors 1-3 are served 7-8am, 4-5 are 8-9am and 6-7 are 9-10am (at least when I was there). Check with reception if you've got breakfast included. Book lunch/dinner slots too, or check if you can show up! Room Service may not do lunch but there are takeaway menus at reception if you want to order something in, they'll place the order for you.
The rooms are cleaned to a good/high standard, there's a safe in the wardrobe and any issues were seen to when reported.
The lift is a tad narrow and takes a while, which might be an issue with big bags or for wheelchair users?
I would definitely stay here again.