I bought this on spec after finding my 450mm spotting scope gave less than satisfactory results when digiscoping.
I was fully prepared to try the lens and return it if the results were less than satisfactory, so tried it on the same mFt Olympus EPL5 as I used last time out with the spotting scope. The scope has a zoom capability which gives rather more image size at a given distance than the 500mm lens, but at the cost of poor image quality.
So to the Walimex lens. This is after a short test on a dull day totalling some 40 or so images.
1. It is a manual lens with a rather slow f8.0 maximum aperture, and will stop down to f32. Frankly with a lens this size you are not going to be snapping moving subjects hand held, but mounted on a tripod via the lens mount, and using a remote shutter release, as I also do with my somewhat more expensive 300mm lens, the results are surprisingly good! Remember with the crop factor of the mFT sensor this gives the equivalent image size of a full size 1000mm lens.
2. I used a slowish shutter speed and got satisfactory photos. I wouldn't don't expect to capture wildlife that doesn't sit still long enough for you to set up the shot. I love photos of waterfowl and with patience I'm sure I'll get some great shots of birds on the water with this lens.
3. The f8.0 maximum is a setting I doubt I'll often use as stopping down the lens increases the depth of field so making focussing less critical. However focussing is easy and the lens has a ring to open the iris to f8 while you focus and then return it to the chosen value for taking the shot. Even at f8 I found the depth of field acceptable for such a long lens.