We spent 4 nights at the Hotel Adonis in May 2008, and had a great time at this hotel, which we found basic, but clean, with friendly staff.

Booking

We booked directly with the hotel - we had a recommendation from a friend for this hotel, but couldn't find it available on any of the standard hotel websites. We had to provide a credit card number to secure the room, but no deposit was required. Note that the hotel only accepts cash payments, so make sure you take enough €uros with you to cover the bill.

Location

The hotel has an amazing location - only 5 minutes away from the base of the Acropolis, in the heart of the 'Plaka' area, which is full of old winding streets with squares full of restaurants and coffee shops.
We did have trouble finding the hotel on the first night. The closest metro station is Syntagma Square on the Blue line, which you can reach direct from the airport. If you walk down the main shopping street (Epmoy/Ermou) next to McDonalds, and follow signs to the Folk Art Museum, you'll find the hotel on the other side of the small church near the museum. I would warn any travellers to Greece that they should have a couple of maps - preferably in Greek and English, since some road signs are only in Greek, and transposing between uppercase Greek (on the signs) and mixed case (on our map) was definitely a challenge!
The hotel is ideally located for the new Acropolis Museum, which we were lucky enough to visit as part of the preview opening (10am-12pm most mornings), and is only a short walk away from the back entrance to the Acropolis, which is generally much less busy that the main entrance.
Near the hotel is a street lined with restaurants, we had some good meals there - strangely enough the nicest meal we had was at the rather suspiciously named 'A Quick One' - the closest restaurant to the hotel, where not only was the food top notch, but the staff were very friendly (they even helped us find the hotel)! Once word of warning - many of the restaurants on this street have a front man coaxing you into the restaurant... we found that restaurants who didn't have to offer you free drinks, discount etc were generally much better - they allow their food to speak for itself!

Rooms

We were staying in a group of 7 friends - 2 twin rooms and 1 triple room. When checking in, we were offered a large room for 5, with a large balcony, which we took. Our rooms were priced on a room basis at €89 for the double room per night, and €170 per night for the 5-person room. Both prices included breakfast (see below).
The rooms are basic, but are very well cleaned and looked after. The beds are fine, but don't expect Four Seasons quality. Both rooms we had were fitted with wood laminate flooring which gave a modern feel. Ask if there are any rooms with balconies available, since this is definitely a plus point if you can get it. The bathrooms are... basic. As other reviewers have stated, there is a rather confusing setup with the shower and toilet, but if you admit to yourself that the shower curtain's going to stick to your thigh a few times during your morning routine then it shouldn't come as too much of a shock to you. The plumbing system isn't up to much in Athens, so there is a general no-toilet-paper-in-the-toilet rule throughout the city, and so you will spot the infamous white bin in the corner of each bathroom. Although this may prove a shock to some, this is standard throughout the city, and the hotel makes every effort to empty every day to prevent any smells!

Breakfast

Breakfast is included in the room rate at the hotel, but if you have illusions of a full English or American short-stacks then think again... The breakfast comprised of Greek coffee/tea, orange juice (actually just strong orange squash), a piece of chocolate cake/roll, 2 pieces small hard toast, and optional cereal (generic unbranded cardboard flakes). Not much to set you up for the day, but the breakfast does have 2 main highlights. Firstly, the staff are extremely friendly and courteous. Secondly, the breakfast room/bar has an amazing view of the Acropolis, even if you don't get the prized seats out on the balcony! You can quite easily while away an hour sipping tea and looking at the view, even if the breakfast isn't the most extravagant.

Conclusion

A good hotel for a short break in Athens, if you don't plan on spending too much time in your room (and don't mind a basic bathroom for a few nights). Not the ideal place for a romantic getaway, but otherwise very good value.