It's a very subjective question obviously, but overall I've been successful while using drugs daily for over fifteen years.
I play and teach music for a living, I get paid very well all things considered. I think drugs are just drugs, if you have some self discipline you can live your life however you'd like to and still use your drug of choice. Things go wrong when the focus turns to the drugs and everything else becomes less important.
I should add that I used to smoke weed daily, drink constantly, and take adderall like candy (I've also experimented with most drugs, Opiates are my DOC.). I gave up weed and alcohol, now I'm prescribed ritalin and take it as prescribed. My point is I've given up alot of drugs over the years so I could continue to move forward. I can function well because I made changes at crucial times, most of my drug use at this time is just what I'm prescribed and I usually take my meds as prescribed. I don't abuse drugs the way I used to, if I did I wouldn't be able to function at all. I'm still not perfect, I go overboard every now and then but work keeps me busy, sane and grounded.
At the moment I'm on subs, ritalin and klonopin and I see a psychiatrist, I couldn't use Heroin daily and function the way I need to but I don't blame the drug for that, I didn't have good enough connections and I couldn't afford the habit. It took too much time to score everyday so I went on subs and feel more balanced now and have more time to do what I need to do. I went through a period where I did way too much of everything and my life and career took a definite downfall. Doing well now though, busier than ever actually.
Overall though for my age I feel successful, I've had some great girlfriends, I've traveled quite a bit, played with some of the best musicians in the world and I work freelance so I make my own schedule. I've had a good life so far but there are times when I feel like I'm walking a tightrope with drugs even though they are prescribed to me. I'm fortunate to have a lot of friends who are professional people that aren't drug addicts, I'm also fortunate to have a great supportive family. My work brings me into contact with so many different kinds of people, from the rich to the poor, this variety helps me keep some perspective. If I just hung out with drug addicts everyday I would be at a disadvantage.
There are alot of factors that make it easier for me to function as a drug addict (people allow musicians to get away with all kinds of shit that would never be tolerated at a normal job), there's always a slight element of luck too, but I do feel I've earned my success. I've paid some dues, I didn't get where I'm at by accident, I worked my ass off learning music inside and out and have played every type of gig imaginable as well as every style of music. I love what I do for a living and feel it is what I was put on this earth to do.
Drugs are just one part of the equation, everyone is different, if your life revolves around drugs and nothing else chances are you will not be a successful drug user. Life has it's ups and downs regardless of whether or not you take drugs regularly.