As far as quitting smoking pot goes, I think quitting it changed me for the better after fifteen years of near daily use... it of course led me to substitute with all kinds of different substances. Looking back at certain friends who criticized me for quitting pot and then getting into other drugs I think the changes widened my perspective a bit. Quitting weed was the first step of many that led me to realize I was self medicating too much, that my so called Recreation was just Self Medication. I still think weed is great for some people, just not me.
I think drugs change people permanently but everything is always changing all the time, people who don't use drugs still change over the years, I think as long as you're trying to evolve it's all fine. Drugs can change people permanently for better or for worse, some people seem to use drugs for a period of time and seem to not change at all. I guess it's all relative. You can use your experiences with drugs as opportunities to grow or as excuses not to.
For me personally I think the drugs that changed me the most were LSD and Mushrooms, they completely changed the way I looked at the world in a way that is completely unique. They redefined the way I see myself in relation to the universe and really made me feel that I could and should live my life the way I want to at a time when I really needed that message.
Well those are my thoughts on the subject, It's all so relative, I think people who go around clinging to their drug days (daze) as a negative time that they wish never happened should try to let go of that idea, accept and own their experiences and move on. If anything drugs have helped me understand how powerful the mind is on it's own. (as I write this on adderall... hard to escape contradiction)