as to those who may not want chocolate...
mix some coffee powder with a little cold water in a second mug, then stir that in instead of the cocoa powder.
pro-tip: when mixing the dry ingredients (flour,sugar,cocoa) make sure they are properly mixed before you add the wet ingredients (really easy to do with a fork)
as for making it in a mug.. hmmm... not enough cake, which is why I tend more towards doubling up the ingredients and use a microwaveable bowl (2 litre type).
As for those that claim the rubbery-taste, that can happen if you have overcooked the cake, as it usually means that it has started to burn