I've used the plugin to check some of those that I develop.
For example, it brought to my attention that I forgot to add a text domain on some labels in a plugin.
It also brought up quite a lot of issues on some very popular plugins :P
You need to install Theme-Check to use this plugin, it isn't mentioned in the description and when I first tried to use it, it produced a PHP Notice and ran 0 tests.
One thing to keep in mind, it can complain about deprecated functions when you define a method that is named like a deprecated WordPress function.
All in all it's a great tool, thanks guys for providing it.