Handheld? What do you think of the ergonomics of the 150-600mm?
Very early days for me Chris, I've only had the camera and lens 48 hours at this point, just long enough to get to grips (kind of) with the menu system and a couple of hours of actual use. The combination can certainly be used handheld but not, I'd suggest, for extended periods. The fact that you can get this kind of zoom range and length in a handholdable lens, albeit with a widest aperture of F8, I think is pretty amazing. To me, so far, the lens feels good in the hand with a broad zoom ring, the zooming is done internally so there's no "tromboning".
I've seen several videos with pro photographers demonstrating the lens handheld but they're holding the lens by the tripod mounting foot! Why? The tripod foot is easily removed for handheld shooting or, I simply rotate the foot so it's overhead on the lens. There is an unmarked aperture ring on the lens which seems to be a "fly by wire" type system. There are good clear clicks as you rotate from one aperture to the next but the ring doesn't stop turning when you get to the widest, or narrowest, aperture setting.
When handholding, I find that my thumb and index finger land naturally on the focusing ring, my palm, third and fourth fingers land on the zoom ring and my fifth finger lands naturally on the aperture ring. To be honest, to this point I've used the lens wide open at all times.
Looking forward, I think it's going to be interesting to see how well the auto-focusing system works with bird recognition and so on. There's a lot to familiarise yourself with regarding auto-focusing options and setups. To this point I haven't used any other fujifilm lens with the camera so, I can't give you a "this lens is more responsive than that lens" opinion.
Another little feature, not related to the ergonomics, is that there is an Arca Swiss profile dovetail built into the tripod foot so no need to fit a mounting plate.