I'll be getting my copy tomorrow. I'll be pitting it against a Panasonic/Leica 100-400 I'm trying out. I just want to see if the Oly is either sharper, faster to focus track OR if it's image stabilization plays more nice with my OM-1 body. I'll let you know.
So here's my quick follow up. Got my Olympus 100-400 today. Against the Panasonic/Leica 100-400 it has the following:
* Faster to lock focus
* More accurate Focus
* Sharper image at ALL focal lengths
* The zoom ring is much more refined, smooth and accurate
* The controls have a solid distinct "click" when in position as opposed to the mushy one's on the Panasonic.
* In absurdly hot weather (atmospherics) it does a better job producing a sharp image, the Panny is fussy about it's temperature shot in.
* Sure, the Oly is a tad heavier but a much more solid build overall. The Arca Swiss tripod mount is another professional touch the Panny lacks.
This Oly may not have a PRO designation but it deserves it more than many.
I had the Panasonic 100-400 and was quite disappointed after trying mutiple copies. I think a lot of people put up with it because it was the longest m43 lens at the time.
I preordered the Olympus 100-400 and have not been disappointed one bit. It also doesn't have the problems the Panasonic had (based on my experience and a lot of online reports).
Fantastic images on this thread and proof positive that this is a high quality lens.
For me the only reason to buy the expensive zoom is for the extra stop of light,which of course can be crucial at times.
I own and love the 300mm f4 but have sometimes thought about a two body and lens combo for some of my wildlife photography as i have the EM 1 mk ii and iii bodies so have thought about getting the 100-400 especially as i quite often use the 1.4 converter on my 300.
I know it's not a dual IS lens (so doesn't work in combination with the camera bodies own IS), so how are people using it. Just the lens IS on its own or turn off the lens and use just the body IS on its own ?
What combination are people finding more effective please ?
I keep going back and forth either using or NOT using the lens IS along with the body on my OM-1. What I'm typically finding much to my surprise, is results are more consistent without the lens IS on, just the OM-1 body IBIS. It doesn't bother me much, it's ALL about results. Does a fine job at some rather slow speeds.
I've had this lens for about a month and at first I had some issues with sharpness with BOF but then I figured it out - it was my techniques. I am just used to shoot ospreys at 1/2000 with Sony and 200-600 or Canon R5 and 100-500.... and getting sharp photos but with OM1 and 100-400 it has been more challenging. Turns out 1/2000 is not enough on the Oly and 100-400! Once I went up to 1/2500 all of the sudden my ospreys became sharp I should not have blindly transferred my settings from how I shoot full frame to how I shoot m43.
I'm not finding 1/2000th mandatory on my OM-1 thus far. I'm not sure what body you're using. I had a R5 and RF100-500L before as well and indeed higher shutter speeds helped. But I'm not finding the OM-1 terribly fussy.
I should not have blindly transferred my settings from how I shoot full frame to how I shoot m43. 
