I played at which can only be described as "unhealthy" amounts from 1999 to 2004, when I started playing WoW, but only casually with WoW since I was somewhat burned from Everquest.
I can say that I definitely gained some things from playing EQ that much. On the flip side, I gave up an awful lot for it. I was in a high end raiding guild, although we were pretty much 2nd Tier, not the trail-blazers, but the next ones in line.
I even ran the guild for 6 months, what a headache that was. It was tough but rewarding. I don't think I've experienced a single day at work that has been as hairy as breaking hate the first time.
I was at the helm for our first successful Trak run, what an experience!
I was a monk, a class that was somewhat undefined for the majority of thier careers, from misunderstood damage dealers / pullers, to overpowered tricksters, to the nerfed, gelatinous, and marginal class that they became. I wore the badge proud. I still have a "monkly business" long sleeve t-shirt and a coffee mug that I bought to keep Zadkiels board going.
Even though I played it a lot more that I should have. EQ had some of the best memories I had in gaming. I remember breaking Hate, Fear, and exploring Velious for the first time. How awesome! Killing Nagafen with 50 people, then coming back to Duo him with a monk/Shaman only two years later. Simply amazing. From Camping Sol B and Guk for the robe of the lost circle, and in turn the Robe of the whistling fist. To pulling in the planes of power and breaking into Castle Mistmoore, I still haven't experienced the rush you got from sitting in the throne of the conquered King of the giants (Tomax?) in any real sense since those days..
I have fond memories of the game, although I'm glad I quit playing it. At some point I had to get one and go do something with my life. What an experience though!