I had been considering getting a 100-400 lens for a while, but it was hard to justify. I already have the first generation 100-300, and picture quality aside, how much of a difference would it be to go to 400? Factor in the teleconverter options for the Olympus lens and it becomes easier to justify going to 560 (1.4x400) or 800 (2x400). I bought this lens and the 1.4 teleconverter, figuring it would be easier to use and less detrimental to picture quality than the 2x teleconverter. Owning a Panasonic G9, the first concern is how well the Olympus would work with it. Some reviewers say it's fine, others say the Panasonic 100-400 works so much better. With the Olympus 100-400 on its own the G9 seemed to perform just fine. I didn't notice any slow focusing or missed shots due to camera shake, even on an overcast day without much light. On this same overcast day I then added the 1.4 teleconverter and didn't find the lens any more difficult to use, other than finding a shot when fully zoomed. To know for sure, I also tried a Panasonic 100-400, and didn't notice any obvious advantages. I am sure shooting style can make a big difference in noticing how well a camera pairs to a particular lens, but for my amateur, handheld bird shots, I didn't have problems with either lens. It's hard to make comparisons with fast moving birds, but I did a test with the more stable moon. For this test not only did the 560 mm equivalent focal length better fill the sensor than the 300 mm lens, but it looked so much better. After seeing the difference in picture quality the cost was easy to justify, even without the reach of the teleconverters. The Olympus with and without the teleconverter and the Panasonic 100-400 were all clearly superior to the 100-300. I also found that the Olympus with and without the teleconverter looked better than the Panasonic. There are many reviews and posts comparing these lenses, with conclusions supporting one or the other, so pick either lens and there is ample documentation to show you made the right choice. Between lens copy variation, camera type and settings, and shooting style, there are plenty of reasons why one may work better for one person than another. For me, my simple moon test (which is repeated regularly looking for eclipses and such) showed the Olympus worked better, and in other testing the Panasonic did not show an advantage, other than size and weight, so I went with the long reach of the Olympus. In full disclosure, I wanted justification for the Olympus because of the teleconverter option, and in my case it did appear to look better. I experimented with the electronic teleconverter built into the G9 and cropping from both brands of 100-400 and the 100-300, but found that the 1.4x teleconverter pictures of the moon looked slightly better than the resized versions without the teleconverter.An undeniable advantage of the Panasonic lens is the size and weight, but it's not as much as specifications initially show. Sure, it's lighter, and that doesn't change, but its length does. The Panasonic is shorter than the Olympus when it is retracted and extended, but it is shorter by 1.5" retracted, and only a little over a half inch when extended. In other words, when at full zoom, the Panasonic is almost the same length as the Olympus. This is an aspect of the size that needs to be considered, though there is no doubt that retracted it more easily fits into a bag. The zoom length may also be related to the oft reported stiffness of the Panasonic zoom ring. The Olympus extends about 2 3/8", while the Panasonic extends about 2 7/8". That greater extension while zooming the Panasonic may account for the greater effort to twist its zoom ring compared to the Olympus. The handle is easily removed from the Olympus by loosening the finger screw and rotating the handle until it can slide off. This makes the lens feel much smaller, but it should still be held firmly with your hand, and not be support only by the camera's lens mount. length retracted; length extended; lens extensionPanasonic 6 5/8", 17 cm; 9 5/8", 24.5 cm; 3", 7.5 cmOlympus 8", 20.5 cm; 10 3/8", 26.5 cm; 2 3/8", 6 cmI also found the Olympus focus ring to be easier to turn. This may seem trivial, but these large lenses require extra support, so it can be awkward to have a firm hold on the lens with a light touch on the focus ring. The Olympus focus ring is light enough that a free finger can easily focus on the bird among the leaves without shaking the camera, unlike the Panasonic which requires greater effort.The build quality of the Olympus feels great, and of course the all metal construction of the Panasonic lens does, too. I have read a few complaints about the durability of the Panasonic lens, but the one I tried was used, graded as "acceptable", and well worn, but still seemed to work as designed without any noticeable degradation to the lens mount or focus speed. Based on my short experience with the Panasonic, testing a copy that appears to have been used for a few years, I wouldn't hesitate to buy one if it were my choice.Several people with Panasonic cameras have also commented on the tightness of the lens mount. Yes, the Panasonic 100-400 feels a little more snug on the G9 than the Olympus 100-400, but I have other Panasonic lenses that are looser than this Olympus, and my tightest mounting lens is an Olympus 60 mm, so in my experience one brand isn't clearly better in this regard. The copy of the 1.4 teleconverter I have fits tighter to the camera than the Olympus lens does, so lens fit is even less of an issue, but for me it isn't an issue at all, either way. I was attentive to this when trying out the lens, and did notice some very (very) light clunking, but found most often it was related to the extended part of the zoom making a slight movement as I walked. The end of the Panasonic lens, though its extension is longer as previously noted, feels a little tighter in this regard.I thought when I tried these lenses that they would feel absolutely huge, but after acquiring the Panasonic 10-25, which was larger than all of my other lenses, and so good that it rarely leaves my camera, I didn't find them to be ridiculously large, at least when retracted. Fully zoomed, yes, they start to look pretty big, and all of these lenses, including the aforementioned 10-25, may make you wonder why you are using a micro four thirds camera (of course, you know that full frame lenses with comparable zoom would weigh even more). The Panasonic does come closer to the size that you might think a super zoom would be on a micro four thirds, and might tempt you into thinking it's a lens attached to your camera, but with the Olympus there is not doubt, it's a camera attached to your lens.This review should have been posted nearly a year ago, and now that I am finally getting around to it, I see there is a newer version of the Panasonic 100-400, which of course brings on another bout of "did I get the right lens?" With my initial investigation, I think I still would have bought the Olympus. The new Panasonic is shorter than the first generation when extended, which of course provides a size advantage, and also may address the zoom ring stiffness as I described above. A huge benefit of the 2nd generation is the ability to use teleconverters, but on the video I saw, the zoom range is limited with the teleconverters attached to prevent the lens elements from bumping into the teleconverter! This means the new Panasonic lens is not fully compatible with the teleconverters, and uses a switch to limit range to avoid mechanical interference. The Olympus can be fully zoomed in and out with the teleconverter. I like to leave the teleconverter on, and certainly wouldn't want to remove my lens to remove the teleconverter if the subject was filling the frame too much and I couldn't move. So, based on a couple of press releases, I still choose the Olympus.