Don't get me wrong, I love my Canon 100-400mm L. In Canon's lens line-up you cannot get more bang for your buck in a lens to cover a wide range of wildlife photography. Sure the f5.6 is a little slow for low-light photography and the dof isn't as shallow as one might like, but with 300mm of zoom topping at 400mm it does a fantastic job without sending one for a home equity loan.
However there is a reason that wildlife photographers lust after the big primes. There is a reason that folks like me look at their prices and feel an empty pit in our stomachs; knowing that someday we won't be satisfied without one. And after saving for a couple of years I find myself sitting in front of the computer, the order filled out; feeling the upwelling of the dry heaves as the cursor hovers over the "place order" button.
A few days later the big box arrives. Almost hesitantly I open it and am soon fitting the beast into the Kirk gimbal head. It feels much heavier than the advertised 11+ pounds but swings effortlessly in the gimbal.
A couple of days later and it's show time at the feeders.
This little fellow mustered up a bushel of "Cute Factor" for my first 600mm prime-time shoot.