Hello!
I think there are a lot of extensions that use something like "if($user->id)". It is the common way since Joomla 1.5. A lot of developers (including me) have used this.
The better way would be
$user = JFactory::getUser();
$status = $user->guest;
(I guess, that has been added in plattform v.11.1... true?)
Seblod sets the Guest status correct. If you are not logged in, $user->guest returns "1" (which means not logged in).
Thomas