On DZone.com there's a new post from Giorgio Sironi about using Composer to install packages/libraries:

The main package source used by Composer seems more similar to the usage of git submodules at a first glance: a list of dependencies on other projects is specified and stored under version control, and upon a checkout these projects are grabbed directly from their repositories.

He talks about what problem the project solves, what issues he's found with it so far (the amount of stuff downloaded for each dependency, the single point of failure of the one Packagist repository) and shows how to get it installed and creating a sample "composer.json" file for an example project.