Funny enough, in the better half's school, the kids are completely banned from having any such thing in the school, and anything "brightly and artificially coloured". Apparently it's made a big difference to the kids.
I've never indulged in them much myself, but I would drink several litres of black coffee a day, so i know the symptoms all too well. Never take them on an empty stomach. Back whe I was a chef, the boss used to talk about TMC, NEF (too much coffee, not enough food), when a poor chef-type's intake of coffee and fags left him gibbering and shaking like a dog shiteeing in snow.