This plugin currently doesn't autocomplete composer commands if there is no composer.json in the current dir. However, the commands create-project, init, search, selfupdate and show are still useful without a composer.json!
app/console by default (if you create a new Symfony project via composer create-project or by downloading it from symfony.com) is not executable. Therefore I get the following error:
sf2 _symfony2_get_command_list:1: permission denied: app/console
_symfony2_get_command_list:1: permission denied: app/console
_symfony2_get_command_list:1: permission denied: app/console
To make command completion work without changing app/console you just have to let php run it.