I bought my 100-400mm when they first came out in late 1999, it has served me very well, suffering the cold of a Helsinki winter and the heat and dust of Botswana, South Africa, Morocco and Gambia, it has only failed me once and that was a result of a bit of a bash, Canon CPS replaced the AF module in 24 hours and its been just a good as ever since the repair.
Contrary to many of the worries that zoom lenses are a compromise, this is a pin sharp lens right through the focal length range, it is an ideal lens for grab shots due to its portability and superb balance. When used with a D60 or 10D it has the advantage of being a 160-640mm because of the 1.6x factor bought about by the smaller sensor size of these camera bodies, in my opinion this makes it the best lens available for flight shots.
The zoom action is smooth and has a lockable ring to fix it at a desired focal length, this is also handy to use for adjusting the tension of the zoom action.
The image stabiliser on it is quite fantastic; with practice it is possible to get pin sharp images at the 400mm end of its range whilst handholding at shutter speeds as low as 1/30th of a second. There are two modes, mode 1 for general use and mode 2 for panning, it is important to switch between the two as it does make a big difference. I have lost count of the times when this lens has got shots that any other non-stabilised lens would have struggled with!
The close focus is a very respectable 1.4 metres so the lens is ideal for butterflies and Dragonflies as well as birds. There is also a focus range limit switch, use this with flight shots of birds (or aircraft and motor sports0) and you will hardly miss a shot.
AF works very fast, and I can tell the difference between how it works with different bodies, on the 1Ds it is instantaneous even with a 1.4x converter, but it defaults to manual focus when using a 2x, the results however are still very sharp with either converter, on the D60 and 10D it is a little less accurate with converters.
The tripod-mounting ring rotates and can be removed; I also use it to mount a lamp on for night photography as I can set the lamp at 45 degrees, leaving clear space for a flashgun mounted on the camera to illuminate the subject unhindered.
I have used other manufacturers lenses of similar spec; none could quite match this one. I think this is the best lens in its class.
Currently about 1150-00 it might seem expensive, but a lens of this quality is an investment, you could get similar for a few hundred pounds less, but the guarantee of getting that unrepeatable shot right first time makes it sensible to get the best.