opiate_eng
Or maybe you're right: they just take away pain. But it turns out a lot of us have existential pain so it works all the time.
Opiates are amazing. They take away a lot of depression. I got up to 50mg of oxy a day, which is quite modest and very inexpensive. It made me happy. Melted away worries. At one of my startups, we got crossed by a partner, almost had to fire everyone. The stress was paralysing - business partner attempted suicide. Me? Start the day with oxy and feel ready to take on things.
Even now, it can change that morning lethargic everything-sucks-why-don't-I-binge-Star-Trek-or-kill-myself into "hey I'm gonna write some software".
The other comments describe it alright - a warm happy dream. It's quite pleasurable even if you're by yourself; you can spend hours just "nodding" off, in and out of consciousness. It's quite sublime. The peace is so powerful, that a few times when I was being self destructive and got to the edge of OD, I noticed but simply didn't care. I'm more careful nowadays, though oral opiates are quite safe if you don't act reckless.
IV opiates step it up a bit. Users talk of a rush. It's similar to an oncoming amazing orgasm x10, then the warmness floods your body and everything's perfect and you might even gasp for breath it's that good. I think part of that is just the IV, and having a huge shift so quickly. For instance, there's a rush when IV'ing midazolam (dangerous, do not use without medical supervision), right before you pass out.
I've gone off opiates a few times. It wasn't as an enjoyable life. The only downsides are constipation (eh, saves time) and possibly lower testosterone (as someone often manic, lower libido is great - but it's easy to get TRT anyways.) I've not seen much as far as long-term damage from use.