I think the good copy, bad copy is crap. I think that the people with bad copies are not using the lens properly. Maybe they expect more out of it. In good light its a very nice lens, and it's a zoom not a prime lens. I have the 300 f/4, then purchased the 100-400 for more reach, and now own the 500 f/4. The 400mm 5.6 is a very sharp lens, but if you want the versatility of a zoom, then the 100-400 is the way to go. The 300 f/4 is a super lens, very sharp and with the 1.4x extender gives you 420 with autofocus, but the extender slows the lens down a bit. For birds in flight, there is no better than the 400, and it's very sharp. So it depends on your shooting style.