Well, WoW still has 7.7 million subscribers (from what I hear, at least) and it's been over a decade since it's release (not counting the beta testing). Here is the wikipage on the WoW that has some sources for that numbers plus some other neat facts.
The game is definitely good. The thing is, not everyone likes the same things. While I personally couldn't play the game past level 40, other people (a lot of people) play it like crack. It is definitely the most popular MMO out there, and it's for good reasons. You just have to be the kind of person who finds joy in those reasons. Also, keep in mind the friend/guild factor. Playing a game with some friends or a guild will make your experience a thousand times more rich and fun. If you play a game by yourself, you will probably get burnt out fast. But with some friends(or a guild) who you can play with daily, you will get a lot more from a game.
So, your questions:
Yes, it is quite popular and is still one of the most popular MMOs out there. Is it worth paying for? That's up to you and if you find you enjoy the game or not. It is definitely a good game (the statistics prove it), it's just a matter of if you enjoy its features or not. I know that WoW is quite casual friendly (a large amount of its players are casual). Also, if I had played the game with some friends rather than by myself (I didn't even seek out a guild, really), I would have probably played it a lot longer than the month or so that I did.

