Hi Barry
I had the non IS version of this lens before I dropped it and broke the AF unit. Parts no longer available - So an expensive lesson is not to buy the older non - IS versions. The non IS was a tad sharper and a bit quicker to AF.
I assume that most of your shots will be pretty well full frame. If you consider you need a minimum shutter speed of say 1/400 sec non IS hand held the IS will give you decent shots at 1/125 Sec. I find that the IS is not as good as Tripod when shooting below 1/100 Sec.
The lens is pretty good at f4 but improves nicely around f5.6 to f8. It is pretty good at tracking large moving targets but I understand the 400mm L F5.6 is much better. But no IS though. A number of Bird forum members have the 400mm and the 100-400mm.
I like the 300 f4 IS because it has a short minimum focus of 1.5m which give good magnification for close up. Also it gives decent results with a 1.4 TC if I need a bit more reach. I keep thinking about a 500mm F4 but as you say if you don't win the jack pot it is unlikely to happen.
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