I've never had regrets playing MMORPGs, but I think that's cause I've refused to pay for them and have only done free beta tests..
I was completing a degree program in network administration and working at a net-cafe when UO beta came along.. I almost failed that course, and NOT buying the retail version of that game is probably what saved me.
We would play our characters day and night, barely sleeping in 3 day stretches.. We'd actually have macro programs playing our characters when we were gone, and some of the distinctions between the best players and the average players were how brilliant your macro skills were.
I recall that someone discovered that the blacksmith's hammer counted as 'mining/crafting' experience, but also a heavy weapon strike. So the smart players would stand in town (a guarded area / not really 'safe') and hammer on training dummies to build smithing and heavy weapons skills.
Course I wasn't really satisfied with getting two areas filled with free XP so I also added two other 'free' actions which was to 'detect hidden' and 'attempt to hide'.
Both those skills dump XP into the 'Ranger' class bonus, and quite quickly, I was one of the very first 'Grandmaster' Rangers in the beta portion of the game. This meant that very few, if any players knew what my talents were.
The ability to just vanish was the first thing that gave me intense joy. I could pickup someone's chest of loot and then just *poof* be totally invisible to them. This left me with a terrible feeling knowing that they had little to no defense against my thievery, so I didn't make a habit out of it, mostly doing it to jerks who tried to muscle their way through the game. It wasn't until near the end of beta that people started bulking up on 'detect hidden' skills, but the instant you 'foxed' a 'hen' everyone in the chicken coop would start screaming that there's a stealth ranger around.. So yeah this stunt got old fast.
Being a grandmaster ranger also gave me affinity with wild animals. This meant I could 'idle' near a deadly bear knowing that few people could disturb me without paying an 'iron price' for doing so..
Due to the confusing learning curve there were a couple occasions where I'd be standing beside a deadly animal and some new player would come trotting up to 'look' at my pet..
"Oh yeah, that, OWCH, is not a, EWWW, pet.. Sorry man. Should I call some next of kin??"
Memories like that, which you can only get in MMOs, is what I found 'made it worthwhile'. I just don't think I'd say the same about a game I'd traded half a year of my life for.. At some point they all seem to turn into this grind of sorts where the magic dries up and you're playing just because of the time you've already put in, not a healthy reason at all.