I played quite heavily for about 2.5 years while I was still in school, in the early days of the game. Probably close to 12,000 hours. But to me, those hours were actually a great investment for following reasons: 1) When I started out, I set out to play in the best global PvE guild. After two years, I achieved that and played with them for another 6 months; and 2) two years later, I had an idea for a gaming related app that took me two months to build and eventually made me $250k. Wouldn't have had that idea without my WoW experience.
Climbing as a random player to joining a top guild showed me that you can achieve anything reasonable in life if you put your mind to it. WoW had 11 million players at the time and for a while I was considered a top 100 player in the game. In a way, it gave me a sped up view of life. And honestly, I'm grateful for that because I probably wouldn't be where I am now!
I eventually quit cold turkey. I'm not sure I regret those four years, I made some great friends, many of whom I still keep in touch with, but it was no doubt an addiction, and it cost me a decent amount of "real life" advancement imo.