I've had the Canon EF-S 24mm f/2.8 STM and EF 40mm f/2.8 STM nearly since they were introduced, and I haven't used them terribly much. The EF-S 24mm STM turned out to be great for infrared shots, so it basically stayed mounted to my IR-converted camera (but I don't shoot IR too often). However, the EF 40mm STM was so close to my 35 and 50mm primes, but without the extra wide aperture, that I rarely mounted it (reduced size/weight are almost never a priority for me).
However, that was before I picked up a DJI Ronin-S this past Christmas. Both of the pancake lenses have been living on my 7D Mark II as they work very well for video. I like especially like using the 7D II + EF 40mm f/2.8 STM with the Ronin-S as the stabilization is even more impressive with the longer focal length, compressed view.