The reason why opiates have such a pull, IMO, is that unlike most other classes of drugs (benzo's/GABAergic/tranquilizers, CNS stimulants..), opiates don't really produce just one effect.
To go into more detail, what I mean is:
Stimulants initially produces an awesome euphoria, and a sense of super human intelligence. However these effects die out within a few hours, leaving you feeling tired, but awake, not to mention depressed and agitated.
Benzodiazepines are debatably recreational, they are euphoric, but in a subtle manner, and many people (though its not recommended at all), mix benzo's with other depressants or use them to ease the crash from bad trips or stimulant come downs.
Opiates however have a little bit of everything to offer, while they're technically CNS depressants, they often produce a stimulant-like effect that actually (initially) wakes people up and gets them moving and motivated. Then as the initial euphoria wears off, the user often experiences a state of calm and well being. So at first dope/opiates seem (at least they did to me) like the all encompassing hard drug, wakes you up, gets you going, calms you down, puts you to sleep.
