In discussion with Shuveb Hussain, who admits that he Killed App Sales By Going Freemium, Marco summarizes it nicely:
Freemium is hard. Its effectiveness depends on where you can put that purchase barrier in your app. Many app types simply don’t have a good place for it.
Pricing is generally hard. Especially, if the magic "free" is involved.
It usually good idea to have free users "pay" for basic service by spreading the word about your application or service (of course, you have to provide them with ability to do so). Once they grow to depend on you and grow the need for advanced features, they would start paying for real. And finding the boundry between basic and advanced determines the success of freemium model for a particular app.