Caffeine is an appetite suppressant, which is bad. As mentioned it will also raise cortisol levels, which will cause you to retain water and store fat from what you do wind up eating. There's at least one positive thing about caffeine that I'm aware of, though. Taking caffeine (the equivalent of eight cups of coffee) immediately after a workout helps rebuild glycogen and convert protein to muscle. However, this study was done with a small number of people (I want to say n < 16). I can dig this up if you want it.
I do better off it. When I get lethargic, I try to find what is wrong with my diet. These days if I need stimulation, I actually just get a little dextrose or sucrose.