I was told that if I bought an upgradable ticket (at more than double the cost of a regular coach ticket), that I could then use my miles to upgrade to business class. I bought the ticket and then I was only able to upgrade on the first segment of my trip both departing and returning. I was not able to upgrade on the 2nd leg of my trip. I could have purchased a regular coach ticket and then paid for economy comfort seats and the ticket would have been $800 less. The way Delta runs the upgrade program is a total scam. I have not received any type of reasonable explanation as to why I wasn't able to upgrade.
I will NEVER fly Delta again.