Hi Kevin,
I think the PayPal IPN feature would be very helpful.
Without this feature, if the user closes the PayPal redirecting page as it is displayed for a few seconds and it displays that the payment has been done, some orders will not be marked as payed.
Moreover, to be able to mark an order as paid, you tell in the user guide that the PayPal autoReturn feature must be activated (and must redirect to the website home page). That implies that it is impossible to use several shopping cart using the same PayPal seller account. That is not acceptable.
Hope this feature will be implemented very soon.