[eluser]Lone[/eluser]
I always like to think of Helper files as you usual 'functions.php' files that were common in the past. But only try to use them sparingly - generally speaking you should be able to fit most of your functions under libraries/models.
Libraries/classes then enhance this your function range as you have another level to call them by rather then some nasty long function names eg.
Code:
// Old way
user_get_details($id);
// CI / Class way
$user->get_details();