Reach isn't the greatest on full frame, but this is one great lens. It is very easy to hand hold and is balanced well. You can hike all day with it and not notice. I use mine on APS-C except for one trial of 6D on owls at dusk (really a trial of 6D AF, which is certainly sensitive in the dark). As mentioned, a monopod can be helpful when staking out a nest or pond, and you can still (awkwardly) move for BIF shots. I shoot at 1/500 or above, faster is better. Good technique, smooth panning are the key to good flight shots, and the perch shots require good to excellent technique for high keeper rate. I shoot bursts (on the 60D at least). The lens is at its best wide open at f/5.6, and I rarely shoot at smaller apertures, aside from the effective aperture decrease that occurs with a 1.4x TC. The 400 f/5.6 image quality is reasonable with the 1.4x TCII but autofocus is lost, so I use the TC mostly for pre-focused stakeouts. 6D as a birding camera is best applied in very low light situations.