I just spend about 14 hours getting PayPal's IPN service working. It used to be that when someone ordered something from our store PayPal sent us an email. Converting those emails to orders on our end was an error-prone manual process. Now PayPal communicates directly with our webserver and the order goes into our database, and from there we approve it and forward it on to our fulfillment house. Nirvana.
On a related note, we are in fulfillment limbo right now waiting for our goods to get to said fulfillment house so they can start fulfilling them. If you ordered something last week it will probably be a few days late. Sorry!