I use it all the time, both as a seller and a buyer.
As a buyer, I like knowing when to expect my package, so it doesn't sit in the mailbox or on the porch all day. UPS and FedEx packages say "out for delivery" when they're on the truck, and USPS packages are scanned at the local post office early that morning, just before (or as) they're put on the mail truck. :mrgreen:
And as a seller, I think it helps reassure a buyer that their package really IS on the way. :)