I think it's got a winning formula, whether or not it was intentional or an accident I don't know, but it does so many things right. It caters to casual players (I can pick it up, guess, draw and put it down) or hardcore players (draw + guess all day and all night), it forces people to provide value back to the player they're against (after guessing you have to draw to play another game) and it has a good micro transaction system.
I've never played mobile games properly before but I find myself drawn into playing Draw Something along with all my friends, who like yours love it.
I actually don't really get the game that much. I've played, it's kind of fun, but half the time people can't figure out how to draw the subject and just write it out ... literally, this happens about 1/4 of the time. But kudos to them for their success. I do question the $200 million and I can only imagine, as another user said, Zynga would try to transition users to their other games in the long term.
What I like about is that everyone wins. Both & the person you are playing with get a reward (coins) for a successful guess. There's no cheating to worry about, no sore losers/winners, etc.
Also, having a little bit of coins may or may not entice you to get more (which means bugging more friends to play, or busting out the wallet).
I've never played mobile games properly before but I find myself drawn into playing Draw Something along with all my friends, who like yours love it.