@sooty mangabey: I'd just like to echo @Jackwow's comments - the lens
is truly superb
I was lucky enough to pick one up about three months ago. I'd hired the Mk I version on a couple of weekends during the previous year, so I already knew the 100-400 range was suitable for me (in contrast to @Jackwow, I shoot mainly on a full-frame body - a Canon 5D3). Having read all the positive reviews earlier this year, I just couldn't resist when I saw the Mk II version available at a decent discount at
The Photography Show in March.
Having looked at my shooting stats on Lightooom last night, I see that, in 22 zoo visits since 25 March, I've shot around 3500 photos (not including the 'instant' rejects, which I delete as soon as they're imported into Lightroom). Of those 3500 shots, approx 96% have been with the new lens. So, anything which I've posted in the Galleries dated 25 March 2015 to date was almost certainly taken with this lens - the only exceptions being general exhibit shots and other obvious wideangle photos.
For me, the main benefits include the stunning image quality throughout the range; and quality is still excellent when using the lens with an extender - I have a Canon 1.4x II which I've used on occasions.
The close-focusing ability is amazing: to be able to shoot -- at 400mm focal length -- a subject which is only 58cm from the front element, is just crazy! (but incredibly useful)

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As @Jackwow mentioned, the build quality (I've used it in the rain without issue), focus speed, and quietness (the IS is certainly a lot quieter than my 70-200 f/4L) are all excellent too. The IS itself is phenomenal: handholding at full zoom, in the less-than-ideal light with which we are often blessed here in the UK, is definitely possible. The lens overall is much better than the Mk I version in my opinion (having used both).
I had been a little worried about the weight compared to my previous 'usual' kit (the 70-200 mentioned above), but I've not found it a problem in practice. I mainly use the lens handheld, although I appreciate having a monopod available to take the strain if I'm staking-out an enclosure for any length of time

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So, highly recommended. Although I should reiterate that I'm shooting on a full-frame body. I suspect that, if I were to use it with a crop sensor camera, the short end would be too long
for me: looking at my stats again, about 20% of the shots I've taken with the lens have been in the 100-159mm range - a range which I would effectively lose had I been shooting with a crop sensor body. However, it all depends on what your usual subject matter is of course.