I've had 100% upgrade success from C+ to FC over the last 3 months. 5 round trips from DTW to LAS, FLL, CUN, MIA, and one more trip to LAS. Diamond / Delta reserve with extremely high spend. I was cleared at the gate on all of those flights except for CUN, where we were upgraded about 48 hours out.
Interesting situation just happened though, which is why I'm making this post:
DTW ---> LAS Flying with my wife, Called up prior to boarding, and told there was one FC upgrade availl, I deferred it to my wife, as I don't mind the exit row in C+. All was fine at the gate, she moved into FC.
The day before our return LAS ---> DTW flight, I get an email from Delta that our 11am flight was cancelled, and I was put in a 2pm flight, wife was put on a red eye. I called Diamond customer service and was told that the agent in DTW split our tickets in order to upgrade my wife when I told them to upgrade her instead of me. The 2pm I was put on was full, so my only option was to change to the red eye with her. I declined that.
We both show up for the 2pm flight, I explain to the lady in check-in about my dilemma. The flight was showing full, with no seats avail. She immediately apologized for the issue and put us both on the 2pm flight in FC.
tl;dr; if you defer your upgrade to a traveling companion, make sure to just accept it under your name, and let your companion sit in the seat you were upgraded to.