9 out of 10 points and recommendedImage quality, build quality, versatilityIt's bloomin' heavy!
(Actual purchase price: 980 UK pounds)reviewed September 7th, 2008 (purchased for $1,729)
This is quite possibly one of the nicest lenses I've ever used on the Canon EOS system. I've been using it with a 40D body for about a month, and it's seen fairly light use, maybe about ~100 shots.
IQ is good at all apertures, but becomes razor sharp at f8 or so. The stabiliser is a fantastic addition, I've hand-held down to about 1/120 at 400mm with minimal loss of sharpness. Good contrast and colour tone, and no visible CA.
Despite its reputation as a dust-pump, I haven't noticed a significant amount of dust on the sensor. Certainly nothing big enough to show at less than f16. Auto-cleaning probably helps a lot with this. I am expecting to see some dust appear inside the lens within the next few years...
If you're going to put this on a 40D, consider a battery grip and hand strap (Canon HS-E1). The hand strap reduces the strain on your fingers a lot, especially with heavy lenses like the 100-400.
I originally bought this for the 2008 Sunderland Airshow (which got rained off, yay) but it's seen use at the 2008 Leeds Half Marathon, and also a little wildlife photography as well.
The downside? It's heavy, and the off-white casing makes you stand out in most crowds. Expect people to come up and ask you to take their photo (with a supertele?).
Last point (and it's a minor one), the shoulder strap included with the case is a joke. It can - and will - cut into your shoulder if you dare use it. Get a large-ish camera bag strap from your local photography shop, or cannibalise one from an old Lowepro or similar camera bag. For nearly £1000, you'd think they could do a little better than that...
Would I buy this lens again? A loud, resounding, heavily reverberated YES. This one is definitely on my "keeper" list. It doesn't get used as much as my 28-135IS (or even the EFS60 Macro), but it runs rings round the 70-300IS-USM.