I just wanted to report my experiences at the Grand Bretagne restaurants for future reference. I tried the roof garden restaurant and can report that both the food and view are excellent. Menu items are 20 - 40 euros.
I also had the breakfast buffet. The breakfast room was closed so it was served in the roof garden restaurant. It is 24 euros, and has very high quality food. There is a huge variety of fruit (strawberries, blackberries, blueberries, etc.) as well as other standard breakfast fare. It is probably the best hotel breakfast buffet I've had.
The hotel has a list of their services that can be bought with starpoints. I'd didn't think any of them were great values, but I'll list them for those interested:
buffet breakfast 1,500 points
3 course lunch or dinner at GB corner 7,500 points
robe pack 9,000 points
body aromatherapy massage at GB spa 10,500 points
back treatment at GB spa 6,000 points
aromatherapy facial at GB spa 9,000 points
As others have posted, this is a great hotel and a great choice for Athens. It is very easy to get to from the airport -- the metro station is right next to the hotel. Excellent service and a great location.