Alan: I used this mostly for bird photography on an extended trip to Baja California. Some of the photos can be seen at http://www.flickr.com/photos/alanharper/sets/72157630258726050/ . I used it exclusively hand-held, often with a 1.4 multiplier. I own a Canon 300 mm f/2.8, which I can compare it to. The advantages of this lens are: lightweight, easily carried in the field, extraordinarily rapid autofocus (on 5D Mk II), unbelievable sharpness. Even tho the lens is 100 mm longer in focal length, it is shorter and much lighter than the 300 mm f/2.8. The disadvantages are: tons of flare in high-contrast (back-lit) situations, not great close focus (can't remember the limit, but I sometimes got too close to birds to be able to focus on them). The flare issue really is annoying. I even saw flare when I photo'd a snowy egret in full sun. There was a "bloom" around the bird, which I needed to remove in photoshop. In general, the flare problem can be controlled by adjusting contrast in photoshop, but this is the major downside to this lens. (It is not a problem with the 300 f/2.8 lens).