Using Symfony2 entity field type one should specify property option:

$builder->add('customers', 'entity', array(
    'multiple' => true,
    'class'    => 'AcmeHelloBundle:Customer',
    'property' => 'first',
));

But sometimes this is not sufficient: think about two customers with the same name, so display the email (unique) would be mandatory.

Another possibility is to implement __toString() into the model:

class Customer
{
    public $first, $last, $email;

    public function __toString()
    {
        return sprintf('%s %s (%s)', $this->first, $this->last, $this->email);
    }
}

The disadvances of the latter is that you are forced to display the entity the same way in all your forms.

Is there any other way to make this more flexible? I mean something like a callback function:

$builder->add('customers', 'entity', array(
    'multiple' => true,
    'class'    => 'AcmeHelloBundle:Customer',
    'property' => function($data) {
         return sprintf('%s %s (%s)', $data->first, $data->last, $data->email);
     },
));