I had a small hot plate mug warmer about 10 years ago. I never really used it, because at home it was not practical to plug it in on the counter, while drinking coffee at the table or in the den. If you were stationary at a desk, then maybe it helps, but you may do better with some type of thermal or double walled cup to help insulate the drink, rather than something to add heat. As the heat output of the plate would be constant but the amount of liquid in the mug would be changing.
I usually pour my coffee into a thermos and pour put only what I'll drink in a few minutes. I still have the thermos from this morning and the coffee is still hot! I just make sure to put hot water in the thermos to heat it up before I pour my coffee in.