When I was searching for a longer tele than my 200/2.8 I eventually got the 300/4 IS + 1.4X TC.
I rejected the 100-400 for it's pull-push design (I hate it but regular users tell me you get used to it finally) and not-so-good performance at 400mm wide open, the 400/5.6 for it's lack of IS, the 300/2.8 IS and 400/2.8 IS for their weight and price, the 400/4 DO IS for the price and the Sigma lenses for fear of dealing with future incompatibility problems. So, by elimination, that left me with the 300/4 IS + 1.4X TC setup. More expensive but more versatile. When comparing it to the 400/5.6 I have two focal lengths, IS (which I think is invaluable in long lenses for a non-tripod user like myself) in both focal lengths, faster aperture in the 300 range, closer minimum focusing distance and still have top-notch optical quality.
I think that IS is invaluable in long lenses. I could hand-hold my 200/2.8 only at a minimum of 1/180 but easily reached 1/60 with my 300/4 IS and 1/90 with my 300/4 IS + 1.4X TC. Yes, I have a monopod. Yes, I like it a lot but most of the times it stays at home. I know, I know. I should always use a support and a monopod is a bare minimum but I am too lazy carrying it with me all the time :-( Thus, as fine as the 400/5.6 is, I will never buy it.