IMO the best all round lens that Canon makes considering the price and it's L glass and has IS. Somewhat soft @ F5.6 in dim light but you can stop down a tad and jack up the ISO. I have another 10D now and the one mounted with the 100-400 gets used the most.
Soon i hear Canon will be coming out with IS II. The push pull zoom has it's drawbacks but being able to carry a 400mm (plus a teleconverter) lens in a Toploader 75A is sweet.
Sab is being humble about the 400 5.6, it's far superior to the 100-400 @ 400mm when i comes to sharpness, CA etc... The advantages the 100-400 has is the range and IS both static and panning mode, also it works well with a Monopod no need to haul a Tripod around.
F 5.6 1/90 ISO 200 100mm Hand held with no post production just resized, about 7:40 pm sun setting behind trees in background.
L glass is sweet.
Oh i never left Nikon, Nikon left me.