We stayed 3 days at the Nosticova Palace and booked some extras in advance (Airport pickup, flowers in the room, bottle of wine) - all of which worked perfectly. While our room was not yet ready upon arrival (we knew it wouldn't be), we were served sparkling wine and a bowl of fruit to refresh - absolutely perfect service. Hotel staff is very helpful and professional, yet not pretentious. Very comfortable. Entrance to the Spa at the sister hotel Grand Hotel The Alchymist is free and highly recommended. While rather small, the facilities are in excellent condition and the decor really special.
Location is also excellent, situated in Mala Strana, the little quarter, all sightseeing hotspots are within short walking distance - Charles Bridge is literally around the corner. Also, with all the embassies around the hotel, the area is very quiet and safe.
It is not many times that I return from a luxury hotel without finding negative points (I must admit that I am rather picky and have high expectations) but the Nosticova Palace certainly had it all and I will definitely go back to this hotel. Highly recommended.