The freemium concept :

The freemium concept is a business model. It is running by offering basic services and the advanced services are paying. The idea is to transform the simple user of the service into a customer. Also, free added to premium gives freemium.
Text link:http://en.wikipedia.org/wiki/Freemium_business_model

we can find 2 online business utilizing freemium concept: skype, meetic, or flickr.

The advantages:

i think that this business allows customers to get free services.

Another advantage is that it will generate more and more customers.

The desavantages :

I think that the fact to get free at the first time and have to pay after will stop consumers to buy. They'll think: why i have to pay now?

Also, it exists some online users who are not feeling secure to pay online. They are asking themselves if it's a secure transaction.

And I think that customers will feel satisfied with a free online service. They won't need to pay for a service they got for free.