Its free account has 2 GB of available space, meanwhile the non-free account has 10 GB and a monthly payment of 10$. Nowadays it’s in beta.
The client application is only for Ubuntu Linux, not for another distributions or operating systems (Dropbox has its own client for Windows, Mac Os, Fedora, Ubuntu…). Also you have a web based client. I haven’t found any problem using it (it creates a folder inside your $HOME), you can select when you are connected to the server and you can use it as another folder in your system.
This service is not free (libre) and it doesn’t offer anything special. It’s for you only if you have Ubuntu installed in your computers, but even in that case, you can use Dropbox.