Same as most everyone else here for me. I see fit to drink copious amounts of the stuff and these days it merely gives me a small edge compared to when I was even 16 years old.
I was, until I stopped drinking it.
In high school, and college, and in fact until about 6 months ago - caffeine did nothing for me. I drank coke and coffee for the taste (until my dentist made me give up coke) and then just coffee. Wonderful stuff. I’d drink cups right before bedtime. I never understood the people who had to end the day with decaf because, so far as I could tell, there was no reason not to pour another few cups of caffeine down. It didn’t keep me up at all. I also couldn’t use it to stay awake if I was falling asleep. I’ve also OD’d on vivarin and jolt and chocolate covered espresso beans (so shaky I couldn’t think or hold my hands still, but couldn’t stay awake) It just didn’t work like that for me.
Every so often, I’d go off it, get the obvious caffeine withdrawl symptoms - but I’d only stay away so long, and when I started up again, I could never tell.
Then about 6 months ago, they switched brands of coffee at work to a kind I didn’t like and I got too lazy to make it on my own in the morning or evening. A bad combination, and suddenly I went from several large cups/day to none. And now that it isn’t coursing through my veins all day and all night, a cup of coffee will actually wake me up. I can still purposefully go to sleep right after drinking it, but if I want to stay awake - the caffeine will help me do so. It’s absolutely bizarre.
Caffeine does absolutely jack shit in terms of waking me up or keeping me awake or hyper. I can drink a gallon of coffee right before bedtime, and I’ll still fall asleep at my usual time. I stay tired whether I guzzle Diet Mountain Dew or not.
When I’m in classes, I can have a 2 liter of Diet Mountain Dew (I only drink diet pop because all I need is the caffeine, not the sugar, and it tastes better anyway) inside an hour, and people are amazed that I’m not affected in any way, other than having to piss like a racehorse. 
But.
Imbibing caffeine keeps me from having the worst headaches I’ve ever experienced. The one time that I went too long without any caffeine (which was, according to my friend, not long at all), I got up to answer the ringing phone. My head went, and I quote, “WHOMP!” and I fell over from the feeling of hammers bashing my skull.
I guess that’s the sign of an addiction. You don’t even get the benefits of caffeine, you just need it to stop with the hurting.
I drink coffee every morning, and it seems to wake me up, though I don’t drink it until I’ve been awake for an hour or so anyway. (That first hour is pretty foggy.)
I have the same headache problems, which I suppose is a sign of addiction: if I don’t drink coffee, I feel it around mid-morning.
I don’t think the caffeine has that much of a stimulant effect, though, since I can drink coffee at night and fall asleep with no problem.
Coffee doesn’t affect me at any time of the day. A few weeks ago I really needed to stay awake and finish what I was working on, so I took a can of some flavored instant coffee mix and choked down the equivalent of 13 cups. I accidentally fell asleep about an hour later.
Caffine doesn’t do anything at all to me. It did once, the first time I ate a handful of chocolate covered expresso beans. After that, nada. Caffine pills, zippo.
Oh, I forgot - I’ve taken a caffeine pill once, and I threw the package out afterwards. Not because it didn’t work or made me twitchy or anything, but because it worked too well. I had to be awake and alert for something important (though I forget what it was right now), and took one of these caffeine pills that my husband had bought for us for that purpose. It wasn’t No-Doz or the other brands that I’d seen before. The stuff felt like what I expected some kind of really good upper might feel like - I felt awake, alert, and pretty happy, though not unnaturally so. No irritability, twitchiness, anything like that. I realized that I liked the feeling too much and that it would tempt me to get hooked on the stuff, so I threw them away.
Maybe it had something to do with the purity of the caffeine, or any other ingredients or something. I noticed that colas or Mountain Dew would produce something close to a “caffeine high/jolt” in me in college at least, but coffee or black tea didn’t. Over time it’s felt like even the soda no longer does it for me other than making me somewhat less tired-feeling.
Caffeine does seem to have an effect but I suspect that the extent of the effects of caffeine is largely psychosomatic.
Caffeine picks me up when I am tired but often rather quickly throws me back down. I end up feeling even more worn out.
[Schwarzenegger-like voice] The caffeine…, it does nothing… [/Voice]
I have never gotten the “awake” effect, but I have drunk so much coffee that my heart was racing like a bat out of hell and my hands were all a-flutter as I passed out from exhaustion.
Now a good sugar high, that’s another thing altogether…
If I don’t have a Diet Pepsi, I feel horrible----lethargic and headachy. I can drink Diet Pepsi morning, noon and night. I often fall asleep with a Diet Pepsi beside me. I drink coffee when I’m cold but it doesn’t keep me awake, either.
I never drank coffee until my last semester as an undergrad. I took a couple of extra classes and stopped having a social life so I could finally get out of school, until grad school that is. Anyway I drank my first pot of coffee around 10 pm and got the shakes then fell asleep. I can drink several cups and go to bed. The only effect is that I will have to get up in the middle of the night and go to the bathroom. Now I will usually drink it in the mornings while I relax before heading to work. Although I can skip it if I run out or don’t feel like messing with it.
Caffeinne: no effect whatsoever, unless I’m using it to treat a migraine. Then I’ll take a couple’a Extra-strength Excedrin and chug a Surge or two and sleep like a baby.
Sugar: I don’t believe that the “sugar high” exists. I’ve never experienced one - even after eating a whole bag of pixie sticks and washing them down with two Dr Peppers.
Medications that make you drowsy: Yeah, right. I’ll believe it when I experience it.
Anesthetics: I wish these worked as well as they’re supposed to - I usually wind up having to have the dentist stop halfway through a job and numb me again. “unn, Dopktob? Shubd I be feebling dat?”
I’m not a doctor but I have been diagnosed with ADHD (inattentive type). My doctor told me that stimulants have a reverse effect on people with ADHD. She said that cocaine can put some ADHD folks to sleep.
Again, not saying that’s what you guys have but who knows?
~t
Caffeine doesn’t really wake me up. Combined with sugar, a couple of Dr Peppers will make me really hyper, but then I collapse and fall asleep.
Gum does tend to keep me awake during French. Can’t go to school without it.
While caffeine doesn’t keep me awake at all, it overstimulates my bladder like nothing else. I suppose if I chomp No-Doz dry in lieu of coffee, my problem’d be solved, but naa… Like coffee too much.
My girlfriend can drink a large coffee at 10 and be asleep by 10:15. Me? If I drink a beverage with caffeine or sugar after 6, I’m awake until 2 a.m.
In college, I had two finals and a paper due one morning and another paper due the following morning. I drank 2 pots of coffee between 9 p.m. and 4 a.m. on the first night, and by 6 a.m. I was freakin out, vomiting and shaking. I called my mom and she thought I was ODing. I ended up staying awake for 46 hours (a badge I now wear with pride). I then slept for about 16 hours IIRC.
So, yeah, caffeine affects me. 
Happy
I consider those who have little or no effect from caffeine lucky. A can of soda within a couple hours of bedtime will leave me tossing and turing wide awake all night, with my heart pounding. I found this out the hard way…thought it was just a bout of incredibly bad insomnia until I figured out the cause.