I really like 300mm but always want more range. I do endangered species work with the state DNR and cropping an Osprey shot at 1000 yards on the other side of a river or lake (they are always on the other side) with a 300mm just doesn't cut it. I'm desperately looking for a 600mm f/4 to carry in the car but when hiking, the 300mm f/4.5 edif hard to beat with an extension tube and tc16a available in the bag. I am sure that a weekend with 150-600 or 200-500 would provide the same level of satisfaction without all the fooling around with lens.