I am an amateur. My current kit consists of a 60D, EF-S 15-85mm, EF-S 55-250mm and 100mm f2.8 USM macro. My subjects are mainly landscape, wildlife and macro. I am generally happy with my 55-250mm but there have been quite a few times when I wanted more reach. I am considering buying the 100-400mm f4.5/5.6L or, possibly, the 70-300mm f4/5.6L. The 70-300mm is smaller, lighter, has better optical quality and better IS but only gives me a 20% increase in focal length. The 100-400mm is bigger and weights more but provides a 60% increase in focal length over the 55-250mm. I was not planning to make this decision now but B&H has the 100-400mm for $1439 through January 7th and, from what I can tell, that seems like a very good price. My big concern is that I have read a small number of comments that the 100-400mm lens inhales dust since the joint where the lens extends when zoomed is not sealed. I have had no dust problems with my 55-250mm or my 15-85mm. I did have an EF-S 17-55mm and it seemed like every time I took the lens out of my bag it had another spec of dust on the back surface of the front element. I won't touch the 100-400mm L if I am going to have a dust problem even close to the problem I had with the 17-55mm. Have others had problems with dust in the 100-400mm lens? Any other comments or suggestions would be welcome. By the way I am going to rent the 100-400mm for a day at the zoo before I make a decision so I will get some experience with the size and weight and the push-pull zoom. Thanks.