Not two weeks after getting a 70-300 lens, I went to a seminar on wildlife photography where it was made clear that you can't really get away with a lens smaller than 400mm. In addition, the 70-300mm Canon USM IS lens does not take a teleconverter (I found out later that you can get Tamron or Sigma teleconverters for this lens).
Well after some frantic saving for our trip to Africa, I traded in the 70-300mm today for the 100-400L IS f4-5.6. The first impressions are that the lens is a monster. It is much heavier than the 70-300 so hiking with it is going to be a chore.... BUT hopefully the pictures will be worth the effort.
I am taking a trip to Maine in the first weekend of October so we'll see.
This Mallard Duck is from the first batch of photos with this lens.