First off, you will only earn experience points and cash up to level 15. You can still play the game as much as you want, but if you want to get more unlocks and other perks, you will have to buy them separately. Naughty Dog has provided detailed infomation about how their Free To Play works. An F.A.Q was also created for people with questions.
This is actually a great idea to introduce players unfamiliar with the Uncharted series and it's gameplay. They broke down the title into parts. Whether you are interested in just the multiplayer or the single player, you get to buy what you want. VG247 reports that there are half a million who are actively playing each month and I don't blame them, the multiplayer is a lot of the fun.
[source: PS Blog US]