I used to but I've never spent much quality time with them.
I think the longest I've ever stuck with one was Phantasy Star Online: Blue Burst (PC). Otherwise, I've played a bit of Ragnarok Online (beta 2) and Granado Espada (Asian servers).
I could swear there was another one but I can't recall which.
Oh, I'm probably thinking about DJMax Online but that was like a max of 8 simultaneous players in battle mode or whatever it was called. :P