The f4 40-150 is a winner in every sense.
The f4 lens runs the risk of replacing both of my other 40-150 lenses. It is not noticeably heavier than the plasticky kit lens and the consistant f4 through the range is handy. It is substantially lighter than the f2.8, as sharp, as fast in focussing and at least as weather proof. It is also $1000au cheaper to replace.
Apart from the performance difference between it and my copy of the 35-100 Pana lens I have respect for, but suspect I got a low end copy, the lens is delivering very much the same performance as my f2.8 with only the one stop of difference.
Something special about the look. Like the 17mm, the Bokeh transition point is hard to pick, making it look natural and contrast is punchy. Unlike the f2.8, I did not have too much trouble with high contrast images, something it shares with the 300mm f4. The f2.8 lens is the master of making the dull look brilliant, the f4 pair are kings of controlling high contrast blow out.
Shooting a junior school athletics day, I missed probably 3-5% of my images all day and put most of those down to me. I was using an EM1 mk2, so the EM1x would have gained me a couple of points. While waitng for the next group of runners, I chased Swallows.
Really very impressive image quality. Bokeh is less troubling than the occassional oddness of the faster lens. A tool, not a consideration.
The reality is one stop is actually not that much benefit.
As a rule, if f4 is not fast enough at all, probably f2.8 will not make enough difference (an extra ten minutes in fading light or a slightly higher success rate with a still too-slow shutter speed). I find that f2.8 for me is simply a matter of depth of field control. If I am struggling at f4, I will likely switch to f1.8, because in most cases I can.
This lens and my 75 f1.8 have a combined weigh and cost that is less than the f2.8 zoom, so overall better options.
F2.8 is stretching it in this light (6400 at 1/500), f1.8 allows for 1/1000th at ISO 3200 and some nice “cut out” depth of field. I go from quite useable, to genuinely high quality.
The f2.8 will be the one lens option for sports under lights or smaller fields, the f4 zoom for brighter light and the 300 for larger field sports. For indoor sports, I will turn to the f1.8 primes.
A news paper front cover image on a slow day (dumped chickens and noisy roosters slipping through the legislative cracks, who knew?).
For travel, the kit lens is still king, because weight and replaceability are all important when travelling.