So, back to the subject:
DL is one of the nicest airlines to non-rev on. The likelihood of being upgraded to Business Class or Elite or whatever it is called today is much higher than at all the counterpart carriers from the USA. Also keep in mind that DL is one of the few that doesn't charge for upgrades. Moreover, upgrading in a lot of International carriers is taboo. There are some Int'l airlines that require the airport agents to assign the last row in economy to NRSAs (provided it is not already being saved for the crew). While your chances of getting an upgrade at DL are high, there is also a chance to encounter a less-than-friendly agent unwilling to upgrade a NRSA (everything is possible).
Many times, I chose to fly DL over my own airline, as my airline (like most in the US) charges for upgrades, but when I didn't get the upgrade, I didn't go on the net and rant about how poor the airline was.