Mark de Jeu wrote:1. Do you have a favorite IS lens(es) for your BMCC EF?
I've used the Canon EF 24-105 f4 L and the EF-S 17-55 f2.8.
The 17-55 is much more useful. It works well on a run n gun set-up.
Mark de Jeu wrote:2. Is there a handheld focal length where IS starts to become important, or is it valuable at all lengths?
I've shot hand held with Zeiss ZF.2 primes. Both 50mm and 35mm. It's doable for dialogue scenes where you are focused on a person talking and there is not much movement but it gets messy trying to follow someone around a room. That's when the IS lens comes in handy.
The wide you go the less noticeable the jitter. If you keep your movements fluid it's also less noticeable.
Mark de Jeu wrote:3. Does IS make sense on a tripod (maybe at long focal lengths)?
As others have said, turn off IS if you are panning but on more static long focal lengths on a weak tripod its helpful to kill tripod vibrations.