Wow?

[ApocaRUFF]

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.

[Phenoca]

Everyone QQ's about WoW so much that I never played.

[ASNZern]

You should still play WoW just to see how it goes.
I like how they revamped the Specialization systems. Now you feel the awesome gameplay and game mechanics since lv 10. And not a lot of those useless Trait like +2% Damage Increase for Skill X and Y where you put 5 points into it just to get it to 10%. (SWTOR has loads of that)

[coldReactive]

I played WoW for 30 days (paid for the 30 days even) earlier this year. I didn't really care for it much at all. Has a Faction PVP System that's forced if you go near other factions, never liked open world pvp anyway. There's a quest in the outerlands on the horde side that has you kill a horde NPC (as a horde) and it auto-flags for PVP without warning. (Doesn't even tell you in the quest.) I had to die a few times via NPC PVP due to me wanting map completion.

I don't like starting a character at a highish level, so I never liked Death Knight.

The pet battles are just, meh, Pokemon clone.

Graphics haven't really changed much either over the years, even with the DX11 update.

Some of the story is still enjoyable, but WoW likes to not focus on the story as much anymore, and I was sad about that.

[Z0MBiE]

There's a quest in the outerlands on the horde side that has you kill a horde NPC (as a horde) and it auto-flags for PVP without warning. (Doesn't even tell you in the quest.) I had to die a few times via NPC PVP due to me wanting map completion.

which quest was that? i don't remember a quest ever flagging me

and to the OP, WoW is still very popular, you'll have no trouble finding a guild and friends to play with. PvP is a bit of a mess at the moment though if you're into that. End game wise, I don't think you'll find an MMO with more to do.