it sounds like a lot of people in here don't get what caffeine typically does.... aka it's not speed and your typical soda doesn't have enough in it to do shit all by itself. and if you are tired already, it will practically never make you not tired.
it's basically a very low intensity upper. If you are already awake it's enough to make you more energetic. If you are already tired it's enough to make you less tired. but if you haven't slept in 36 hours it isn't going to be able to do a whole lot to keep you going for another 12 after that.
the biggest "benefit" of caffeine comes with sugar. the two together have slightly stronger effects, but again your average soda isn't enough to do much. a power drink (especially with like 50g of sugar or more) is generally enough to get most people moving, but also is usually not enough to do a lot more than mentioned above. Make an awake person quite a bit more active, and a tired person a fair bit less tired. Of course with caffeine there is usually a less harsh period when crashing. With sugar added the crash is usually a bit worse. You are not going to fall right asleep, but your body will be noticeably more groggy than when you took the caffeine sugar.
as mentioned earlier in this thread, the people who say it has no effect on them are usually either not taking enough caffeine/sugar to do anything in the first place, or have much different expectations on what it is supposed to do. there are of course some people who it might have little effect on, but most just don't understand what it is actually doing.
edit - and the people who say it does nothing for them except cause headaches/withdrawal when you don't have it, look up chemical tolerance. If you really want an effect from it just drastically increase your dosage.
