I only stayed 2 nights at the Mandarin but it was a very comfortable hotel. My room was very nice; large windows, comfortable bed, nice bathroom and a decent internet connection. The hotel is located about 2 minutes from an MRT station. From there you can go to most places in Bangkok if you know the transit system.
The staff was very friendly and helpful. My only issue was they didn't provide me with internet connectivity when I checked in and they didn't tell me where the gym was located. Also, the gym could be better equipped. There is a huge decorative sofa in the middle of the gym. No squat rack, no barbells and no dumbells over 20kg. There were a few cardio machines , 3 exercise machines (lat pull, leg extension and seated chest press) and one bench. Typical "fancy" hotel gym.