I've never bothered to backorder. First of all, many domains are on auto-renewal meaning the original owner can renew and it will never become "deleted." If they don't renew, it takes about 90 days after a domain has expired to truly be deleted (and become available for purchase).
I'm not sure who gets priority on backorders, but I would assume it's by date ordered?
I know several people who were successful in their backorders..through GoDaddy. Hope that helps!