People here are not really objective? eh?
Is World of Warcraft a good game? Well as you said 10 million people play it, and its quite the amazing world they have created. Tough if the game is good or not depends on your game style.
I am a hardcore instant action fan. I can't rely on a 10% block rate or fireballs that curve after me even tough i change my angle from the cast be 180 degrees. And my last fantasy/RPG before WoW was Oblivion, the almighty of instant medieval magic game play.
BUT, on the other hand. The game has an amazing growth possibilities for leveling a character. The class build is strict and precise, yet races are mostly for looks, and the game is greater at creating a false social atmosphere! You know that behind every char there is another human being, but who knows? Might be your grandma, or that weird guy from your biology class that tried to give mouth to mouth the the frogs.
But there is no answer to this question. A game is what you make it to be. To take an example, look at the game i worship: Oblivion. Its in my humble opinion the best game i have ever played, yet people who played Morrowind first say that Oblivion is the biggest mistake Bethesda ever made. So take a 10 days free trail and try it out. But also remember that the game don't start until level 50-60.
Small tip at the end, get a "RAF" Recruit A Friend. You'll get a way faster leveling experience and you will have a person to turn to when you need help. And also: No more then 8 hours of WoW pr day, not even in weekends! That is and order! And if your grades drop or you forget the smell of "outside" then its time to stop.
A game is like a book, well not exactly. But you cant judge a book by its cover and you cant judge a game by other peoples opinions! Read the book.. i mean, try the game and make up your own mind. You are the best person to judge if you will like it!
(sorry for the length, its late and im bored cause im not playing WoW...)