The 100-400 has IS and is amazing for the loot(under $1500 easily and you can snag one for $1k if used doesn't spook you).

The Canon 500mm F/4 L is THE King. It takes the 1.4x extender exceptionally well,is lightning fast,has mind blowing IS capabilities and grabs a fair amount of light. Downside is that it is fair sized,but IMHO it's IQ is without peer.

Here's one mounted on a 5D with the 1.4x and that was from the Monster Polar Bear Safari. He could shoot birds in flight,with our bucking into a pretty good chop at 25 knots from a flat bottom landing craft. Admittedly the light was favorable,so he had shutter speed on his side,but it's IS is killer.



Another from last night and it will keep the girls guessing.(grin) Had an eye-winker of a shaft of sun poke through the clouds and I fish-hooked him to garner it to my favor.