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.
Pain relief. This is a big one for me, since I've had back problems for a number of years, but opiates provide a level of relief I don't get anywhere else. Without the ability to at least temporarily escape from my back pain and have some level of control over it, it would be much harder to deal with.
And cons:
Money! I shudder to even think of how much of my money has gone into opiates over the last 2 years. Of course even before that I would spend a lot on drugs, but opiates took it up to a whole new level because I'd use whenever possible, instead of restricting it (mostly) to weekends like I did with other drugs.
Withdrawal. I've fortunately managed to avoid a serious habit, though I've had some extensive binges, but even after 4 or 5 days of consecutive use (nevermind a week or a month), I feel pretty horrible for 48 hours or so. Aches, depression, fatigue, and forget even getting out of my room and doing something productive. Fortunately I live in a country where codeine is OTC, so if I have to I can do a CWE for a day or two to bring myself down a bit more smoothly.
Danger. I've been fairly fortunate in avoiding overdoses (other than one time, fucking fentanyl

), but even as careful as I am when it comes to measuring doses and poly-drug use, there's always some slight risk that something could go wrong, or I could fuck up.
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.