I stayed here for 6 nights on a work conference. Excellent hotel, good location, about 1 hr drive from the airport. The city centre and the beach are both easily accessible via tram system, subway system is also very easy to use and efficient.
Rooms are very clean, and spacious and comfortable. The rooftop is a must for after dinner drinks, with a great view of the Acropolis at night.
Fantastic conference facilities, and well organized. This is a luxury hotel, and it lives up to it's class. The only downside is that I believe it was a bit overpriced, even for being a 5-star hotel. Personally, I wouldn't stay here again if on my own budget, but if the cost is covered by your company or otherwise, this is a great place to be.