Caffeine - good, bad, ugly?

As I sit here happily sipping a Monster Rehab, I am wondering - what are your thoughts on caffeine intake?
Obviously there's a lot of opinions on this subject, with some people swearing by caffeine to boost their workouts, and others who are highly sensitive to it and get sick from even a little bit of caffeine intake. And obviously, many caffeinated beverages can be high in calories and/or very expensive.

I really enjoy caffeine. I'm almost uncomfortable admitting that, to be honest - but I love the burst of energy, especially since my endurance often feels so low normally. I am probably addicted to it, I realize that, too (though I don't tend to get headaches if I miss my morning coffee). And of course my tolerance to the stuff is pretty high - though on a typical day, my intake is about 2 cups of coffee.

I don't often buy energy drinks because they are expensive, but when they're on sale or I feel like I can splurge a little, I'll tend to pick up something low-cal (the Monster Rehabs are 10 calories per serving, very doable!). Out of curiosity, though, I just looked up the price on liquid caffeine additive - with the ability to add it to anything, including 0 calorie beverages, it ends up looking very economical, both for my wallet and my calorie budget. But the idea of buying that then feels "dirty" to me, like I'm some kind of degenerate drug addict - am I just being overly judgemental of myself, or am I right about the slippery slope of caffeine use? Or maybe there's a huge range of grey area in there.

So, let's talk about caffeine! How do you feel about it? Do you use it, or do you avoid it? And where should someone draw the line?