NOTE from 1.5 years later: This is not best practice. If you want a testable controller or write a reusable bundle, you should take the effort to inject the services you actually need, instead of extending the symfony base controller. For rapid development however, i would still recommend this as the base controller methods are very convenient.

I often want to have my controllers be a service so you can inject some information. But at the same time i like to extend the base controller Symfony\Bundle\FrameworkBundle\Controller\Controller to have the convenience methods like $this->render. Now if you just create a service from your controller, you won't be able to use the methods anymore. You will get exceptions about calling get() on a non-object.
What you need to do is inject the service_container to your controller manually.

In XML, this looks like this:

<service id="liip_vie.viecontroller" class="%liip_vie.viecontroller.class%" public="true">

    <argument type="service" id="service_container"/>

    ...

</service>

In YML, it looks like this

services:

    liip_vie.viecontroller:

        class:     %liip_vie.viecontroller.class%

        arguments:

            - "@service_container"

            -  ...

And your constructor will look like this:

use Symfony\Component\DependencyInjection\ContainerInterface;

...

public function __construct(ContainerInterface $container, ...)

    {

        $this->setContainer($container);

        ...

    }

Hope this helps. At least i always forget how to do it. Now i know where to look it up when i need it next time :-)