I was on a MSP-BWI flight recently where C+ was sold out when I bought my ticket, more than 5 days before departure. Someone therefore must be buying the extra 3" of legroom in C+.
Given that there are almost no upgrades to F anymore other than on regional jets, where I can't buy a seat in F for a small premium over coach, I'm looking for basic economy, since I have short legs and don't need the extra legroom.
On my most recent trip, DCA-DTW-PDX/rental car/SLC-DCA (which got diverted to RIC due to storms in the DC area), there were no upgrades processed on any of these legs.