My personal experience: no drug is addictive. Not a single damn one. Furthermore, no form of amphetamine or opiate was ever even remotely fun the first time. In fact, my first experience with opiates was horrible, and I have no idea why I kept trying them.
But I'll tell you one thing that IS addictive: running from your emotional problems. If you start using drugs to mask your issues, and you never confront those issues, then you'll most likely become addicted. You won't be able to break your addiction until you deal with the underlying issues that drove you to drugs in the first place.
It's sort of like dieting. "Losing weight" is such a farce. Your body is a reflection of your mind, so when your body goes to shit, it means your mind went to shit first. Your body is sort of like an indicator gauge of your mental health status. If you try "losing weight" simply because you're fat, then obviously you're going to gain the weight right back. To truly morph your body into a slimmer form, you'll have to work out whatever underlying problems made you fat to begin with (i.e. food addiction and/or lethargy, caused by depression, or ???).
EDIT: I re-read your post, and it's fairly obvious that you're not the type to get addicted. Mainly because you sound like me, and I don't have an addictive personality. I've always known when it's time to stop. I may get carried away for a month or two on end, but there's always that point where the angel on my shoulder makes an appearance and says "Dude, really? Do you even know what you're doing anymore? C'mon, let's go. It's time to stop and get your shit together." Like I said, addiction is a state of mind. It's caused by you, not the substance. Your specific drug of choice doesn't really make a difference.
My advice though, would be to NOT try heroin, unless you're experienced with opiates and have some bit of narcotic tolerance. It's too strong of an opiate to start out with, and you'll no doubt spend the night simply feeling dizzy at first, then it'll turn into profuse vomiting, followed by severe constipation. My brother is opiate naive, and I gave him a tiny 4mg bump of hydromorphone last week, and he couldn't shit afterward for 3-4 days. He was also praying to the porcelain god before night's end. But if you ARE opiate experienced, then by all means, try it out. I can tell you're smart, so like me, you'll also probably worry about the shit they cut street drugs with. I've never consumed any street drug without purifying it first, so you might want to look into it. I would never in a million years buy a drug off the street and consume it, not knowing exactly what's in it. The very idea makes me shudder. Especially with heroin, and especially especially with heroin with no tolerance. That's why people are always ODing on heroin; they either a) took a break from it and didn't know that their tolerance had decreased significantly, or b) they got some that was extraordinarily pure and dosed much more than they thought they were dosing. For a first-time opiate user, trying heroin, if it was pure, you'd need almost a speck of it. Like an amount equivalent to the ball in the tip of a ballpoint pen. So with an amount so small, purity could mean the difference between a buzz and respiratory failure.
Also, the reason opiates are what they are, with the addiction and withdrawals, is because it's not a drug you do only occasionally on the weekends. For most people, it's a drug you do every single day, without break. It's just sort of sickening if you do it every now and then, but after a few days of straight use, that's when it starts to really become enjoyable and you start to appreciate it. Then, before you know it, it's been 6 months and you've done it every day...
Also thought I'd add this, in case my biochemistry is vastly different than yours: I don't find any kind of "upper" to be enjoyable. I dislike coke, and strongly dislike Adderall, Dexedrine, and meth. Never tried E. The only drug that truly brought me euphoria was shrooms. Shrooms give me a feeling comparable to orgasm, and it lasts for hours. Oh, and one more thing: I've never experienced an opiate "comedown." I've never felt any sort of negative after-effect (aside from withdrawals after a few long binges). They don't produce any comedown or hangover that I know of.