Strictly annectdotal, but I think the "unlimited upgrades" work well on DL for lower and mid-tier elites as the Silver and Golds I know are usually never sitting in Y. Sorry, but this is bum information, Silver's on DL are almost always sitting in Y in my experience. When we are upgraded it's most likely due to light loads, off-peak flights, or flights with a lot of leisure passengers. I have a coworker who's Silver and a Million Miler on Delta and he considers himself lucky to get an upgrade when he flies them (and he gets the Million Miler tie-breaker over other Silvers). Our recent trip on DL was ORF-ATL-MCI and neither of us was upgraded on either flight.