Caffeine is one of the most useful supplements I use...it blocks adenosine (the hibernation molecule) and perks me right up temporarily.
I definitely have to watch it though because it is a fine line between just enough and too much.
Supposedly green tea is better at smoothing out the highs and lows and I think that must be true because I can drink a lot of it and not notice the caffeine at all - either the positive or the negative on energy - independent of all the good EGCG.