I only got deep into bird and wildlife last summer when I purchased Canon's EF 400mm f/5.6L. That's a fine lens and I thought that the image quality was very good, but I longed for more "reach" and a faster aperture that would autofocus with my 1.4 teleconverter.
A professional photographer freind let me handle his EF 600mm f/4L and his EF 500mm f/4.5L (both are older models) and the added length and weight of the 600mm focused me totally on the 500mm for my bag. My friend is using his 600mm on a full-frame camera while I'm using my 500mm on a 1.6 crop-sensor Canon 7D. On that sensor the 500mm gives a field of view equivalent to 800mm (1.6X500). When I add my 1.4TC II the equivalent focal length is 1,120mm!!! That's amazing power for a lens that will fit in my LowPro backback.
For stunning pictures at all shutter speeds and shooting over an extended periods of time, a stout tripod and gimbal is suggested. I'm using the eight-ply, carbon fiber Induro C414 tripod with an Arca-Swiss Z1 ballhead and the Wimberley Sidekick half-gimbal. This setup is extremely stable and you can turn off the lens' IS. I leave it in mode 2 for panning. That said, this lens yields very sharp images when handheld. The IS is truly effective. To see it, just focus on something stationary without pushing the shutter button to activate the IS. You'll see the image moving and shaking, then push the shutter button and the subject almost "locks" into place, as if it's on the tripod. Yes, there's still small movement, but at 1/1000th to 1/2000th shutter speeds typical in bird photography, everything is sharp. (Remember the old rule, for every 100mm of focal length you need 1/100th second of shutter speed, so the minimum with this lens would be 1/500th, even when shooting something that's not moving. (Factor in a crop-sensor that might be 1/800th second). At those speeds the IS gives very clear results.
The auto focuse of this lens is really fast and mostly "snaps" instantly into focus. You DO need to be aware of the subject distance and move the switch for a close subject and remember to move it back for a distant subject. Be aware of your camera's AF mode. For instance, for birds in flight I use 9 sensors in a square box in the middle of the sensor. For birds in the bush, I use single point in order to look past twigs and such. For things like a passing boat or car it hardly matters which mode your camera is in. When used with my 1.4TC II, the AF snaps seemingly just as quickly.
My Flickr site is [...] Cut and paste if you want to see examples, Tagged "EF 500mm f/4L IS"
I see much superior image quality to my EF 400mm f/5.6L. A big part of this is the effective focal length over 1,000mm when used on a crop-sensor camera and with the 1.4TC, which gives you a frame-filling subject. I still need to crop many of my bird images, but it's mainly for compositional values. This lens is large and does attract a crow (probably the biggest negative) but it gets the job done.