Personally, I think it's a pretty awful idea. Most free games that use this system (there are some exceptions, such as TF2 and some others) are plain bad. Just horrible.
And then they have the *brilliant* idea of implementing micro transactions. What do you get? A s**t load of angry people because the players who pay get a 1-hit kill assault rifle.
And that really bothers me. It ruins all the fun of working hard for weapons, items and stats of both the buyers (they level up twice as fast, etc) and other players. (what's the point of leveling up if you know that if you don't play you'll never be able to beat premium guys?)