I've been through the whole lot! Started off with a Sgma 400mm f5.6 the build quality of which was dubious to say the least, (this was a good few years ago I hasten to add). Replacing it with a Canon 400mm f5.6 was the best decision I ever made. But the views of the professionals and advice from those in the know, eventually persuaded me that my wildlife photography would benefit from an f2.8 lens. It didn't! SO large, SO heavy, very impracticable, - shooting at f2.8 I found that the depth of field was so limited it was necessary to stop down to.... f5.6! Two years of huffing and puffing with the thing I traded it in for the new 400mm f4 DO lens. At last here was a lens I could use wide-open and still retain a resaonable d.of f. Unfortunately even the muscles I'd developed using the f2.8 hadn't the resiliance to keep going with it for long though, (and the hood was like a giant waste-paper bin)! - so I traded it in for a 100-400 f5.6 zoom. I soon got used to the push/pull mechanism, (never had any trouble with dust in it although I used in in both India and Kenya over a period of a good few years), it was smaller and lighter than my last two lenses, nor did I notice any difference in the IQ. A year ago, (getting fussier over IQ now), I traded it in for another f5.6, - there is definately an improvement in IQ but I do perhaps, miss IS. So over a period of some 20 years, a complete circle (shows how old the f5.6 is)! On a practical basis, I used it on my last photographic safari in Kenya with a bean-bag propped over the base of the open window of my jeep, so IS wasn't a necessity, but for most of my general shots I used a Canon 70-300 f4-5.6L IS which I love to bits. If you're using a crop body (7D or similar), I'd advocate the 70-300 IS zoom as a first choice. If you have F/F, the 100-400 zoom is fine, the 400 f5.6 is sharper but no IS, - take your pick!