My experience is strictly pharmaceutical and mostly post-surgical at that.
Several years ago, I had a hip replaced due to trauma. I've seen one of those done on cable surgery programs, and I blanched at seeing the joint literally forcibly dislocated and the patient hoisted up like a side of beef ready to butcher. When I came out of my surgery, I wanted to just about die. The pain was the most all-over, deep and intense as any I've ever had, and I've had fair bit of orthopaedic (that "ae" is for our British friends

) injuries and trauma. I had a bolus of IV morphine immediately after being put in my room, which I think was 30 ml, but whatever dose it was, the pain tore through it and kept on laughing. Pre-surgery I had requested (God I love modern medicine!) an on-demand morphine drip, which eased drop or so into my IV as quickly as every 5 minutes upon my punching a button. That was hooked into my IV and I started pushing the button like crazy.
It took about 4 hours, but gradually the pain eased, although I was pissed they didn't hit me with a larger bolus. But after my pain got under control, I could think about what a Godsend the IV morphine was. I was lucid, even stimulated, but not loopy or goofy. It was marvelously relaxing and euphoric, and I was still sharp enough to carry on pleasant conversations with several visitors only 6 hrs after they quartered me like an Angus beef. I was afraid and anxious about nothing. I began my rehab 18 hours after my surgery, using a walker to just about run down the hallway. I was truly a morphine hero and loved working my rehab hard. My family could hardly believe I'd had surgery.
I was on the IV for 48 hours, and then to IV Demerol for 72 hours. It's my own personal reaction, but that stuff is like witch's brew to me, and gives me startling and disturbing dreams, even hallucinations. But the IV morphine made me understand why people become needle junkies: it was that good.
I've also had a number of minor surgical procedures in which I was hit with Sublimaze (fentanyl) pushed in my IV. WOW! So THAT'S what a rush is like! But it's over in about 30 minutes to an hour. It's a good thing I don't work around fentanyl; I doubt I could keep my hands off it.
I'm may be having some problems with failed back surgery now. I have an interventional pain management specialist, but I have limited his work (other than some Norco 10/325s over a 4 month period) to some cauterizing of spinal nerves. I can tell from my discussions with him that I could proably ask him, and he would let me choose between Duragesic patches, methadone, or oral morphine. I'm not ready for that; I've enjoyed the so-far necessary pain therapy too much to blithely put the hammer down. But I understand the attraction of the needle and the rush.