This lens was to replace my very low 75-300mm mk 3 (for the price that lens was not bad, but left alot to be desired). I bought this lens as I needed more range and better IQ. Its hard to find a cheap alternative to 400mm, because mainly you have the 100-400mm and the 400mm 5.6, after that you have to pay alot more. The 100-400mm was my first choice because of the range it provided, and having so much availble is awesome. This lens is different then most because instead of a twist zoom, this lens requires you push and pull it. I find the push/pull design to be very effective, for it allows you to zoom very fast. There is a friction ring which allows one to set how much force is required to zoom the lens, for you can make it loose and have it zoom quick, or the opposite. The friction ring also will let you lock the lens in place at a certian mm. Shooting with this lens is a joy, but good lighting is needed, and if not you will need at least a monopod. IS helps alot on this lens, but don't think of it as a "cure all", for this is a 400mm lens at the max and user hand shake is going to happen. How sharp is this lens? Well my copy is sharp at f5.6 and gets better stopped down to f8. It also depends how much you plan on cropping, often their are times I crop tp 75-100% and still get great results. The build quailty is amazing, very sturdy.
All in all this lens is a beast! You get great focal length coverage in a 4X zoom. The IQ on my copy is great! You can't simply go wrong if you by this lens.