Hi all. I just received my Sigma 150-600 Sport from a local camera store in Cincinnati, OH/USA. I did some in-store testing tethered to a laptop (I'm a commercial photographer and almost always shoot tethered). The lens was sharp enough to make me follow thru and buy it. Got it back to the studio, threw a Nikon D810 on the back, slapped some light/studio strobe on one of my dogs and shot away. At just about the minimum focusing distance at 600mm at f9 I got the attached file. Please note you're looking at the center part of the frame. I did sharpen in Photoshop at the default "Smart Sharpen" setting. Quick note that Adobe recommends "Smart Sharpen" as the best way to sharpen. I've never shot a Nikon 600mm f4 so I can't compare but I have to say that I'm pleased with the Sigma. My Nikon 300mm f4 is sharper but the Sigma will do it's job if I do mine. Please note that at minimum focusing distance at 600mm, especially wide open, if you miss focus you miss big time. Same holds true in any high magnification situation. Oh, yes, the build quality is very, very good and regarding the weight, yes, it's noticeable but keep in mind that for a 600mm, though a 6.3, it's still about half the weight of the Nikon 600mm f4; maybe that means the Sigma is actually light weight?