It is becoming very popular. From MMO's aspects it seems that once the target audience loses interest in an MMO it becomes free to play as a second wind of trying to squeeze all it can in profits before it goes offline like Matrix or Tubula Rasa. I mean the positive is you can play the game for free with no risk of purchasing it and turning out its a bust. The only drawback is the fact that people can buy there way to an advantage over you (same could be said about TF2). It does not mean that people who purchase points are gaining access to exclusive weapons or items you can not get, just means they are able to get them faster unless you put the time into the game yourself
In any case, I do not see the negative of purchasing items (keys, items, my strange sapper that I want so bad) since I figure 30-50 bucks invested in the game anyways if I like it so why not put that into items or points