Four flights earlier this week: MCI-SLC-OAK and OAK-SLC-MCI. I'm a Silver, with Delta CC and MQD waiver earned. On all four flights, I paid for Comfort Plus on a W fare. For upgrades I was ~18th in line for 1-3 available first class seats.
I should note, I received an offer for a first class upgrade MCI-SLC for only $45 at check in. I didn't take it.