Primarily for business-class trips to Europe, then domestic trips for relatives and very occasionally when I need to travel domestic at a specific time but the prices are too high. I've managed to get tickets at the lowest mile level for almost every trip on NW, UA, SK and AA, but suspect that I will no longer be able to accomplish this on DL.

Oh, I upgraded a couple of NW Europe flights with miles on a B fare several years back. Forgot about that.