Speaking for myself, the pro's:
Euphoria. This one everyone can relate to, I think. I've tried a hell of a lot of drugs, and none of them match a opiates for providing immediate and overriding happiness (except arguably meth, but it always felt like you pay a lot more for the meth high than you do the opiate high). No matter how pissed off you are, no matter how tired or bored or unhappy you are, opiates will fix it, and sometimes we all need a bit of self medication, or something to look forward to after a long day.
And cons:
Effects on personality and lifestyle. This is the major one, and I think the one that a lot of people overlook, since it's slow and subtle, but opiates tend to override a lot of other aspects of someones personality. They inspire a particularly obsessive devotion in their users, to the point where it begins to blot out other parts of their life. You start pouring all your money into opiates, and you have less for the other things in life you used to enjoy, and possibly even just for the basics of life. It's easy to neglect hobbies, aspirations, relationships, responsibilities, because why work on those things that might make you feel a little good, or make you happy down the track, when you could go get high and feel 'really' good, right now. Why spend the time and money to go out with friends, or to a concert, or on a holiday, when you could have just as much fun sitting at home with a bag of drugs and a needle? Opiates make you feel better than anything else, but they ask for everything else in return.
+1 good post.