Once... so far. I just got bored having to grind out stuff for my professions (spending hours doing nothing but killing the same enemy 20 bajillion times for something that drops barely 5% of the time is not my idea of fun), as well as not being able to go raiding because of my schedule. And on top of that my friends stopped playing around the same time, so I just quit.
However, I am now back in. With two 80's. My brother transferred my already 70 mage to his account and got it to 80, and also got a Death Knight to 80 (my brother loves WoW) But his account got hacked, so nearly all of his gear was deleted. So now I get to run instances to get better gear, which I don't really mind, because they're kinda fun, especially the raids. Also, WotLK added better graphics, better tools, and new art design/story* for Northrend is much better, and far more interesting.
* By story I mean certain instances are part of a chain, which advances a story (Brann Bronzebeard/Halls of Stone>Lighting>Ulduar, Forge of Souls>Pit of Saron>Halls of Reflection> Icecrown). I also really like the Ymirjar/vikings.
Edit: However, I do not have my own account, and I am not the one paying. Would I pay $15 a month (or buy a subscription card)? Yes. But Then I would have to transfer my mage back to my account, and possibly the Death Knight, then change the Dks name. All together, thats over $60, which I am not going to pay.