Its a great lens, the IQ is excellent for a zoom with this sort of range and the build quality is just what you'd expect for this sort of money and 'L' series. Ok, the IQ is not as good as the Canon 400mm primes, but you can take this out for a walk, and it zooms :-)
The trombone zoom is easy to use even if you're used to the 'normal' twist zoom. It's supposed to be something of a dust pump and to deposit dust on the sensor, but so far I've had no problems. Focussing is generally quick and there's little or no 'hunting' for a correct focus, particularly if you have the minimum focus distance buttons set to help the lens.
The IS is effective, but as it's first generation, there's only two stops available and you need to switch it off if you're using a tripod. You can switch it for panning shots. After a couple of afternoons out with the lens I think I can say it also eats the battery on my 400D so I make sure I've got a spare if I think I'm going to be using it a lot.
The f4.5 min aperture means you're probably going to be using it with ISO 400 on anything but the brightest day if you want to keep the shutter speed up for hand held shots or moving targets.
It's heavy. Not too heavy to prevent me from taking it out for two or three hours walk or using it hand held without too much problem, but it might be a good idea to try holding something of similar weight if you've got doubts.
It's quite long. Stood on its nose I can just fit it into the Lowepro Nova4 bag I use. If your bag isn't as high, you might have problems.
There's a 'friction' ring for adjusting the stiffness of the zooming action and you can move it from no resistance at all to completely locked. This ring sits next to the focus ring and there's always the change you'll turn this instead of the focus, but as I'm almost always using AF it's not been a problem to date. I always make sure I've got the lens locked when I put it in my bag, but I've had a number of occasions where it unlocked itself and has fully extended as I've removed it from the bag.
Then carrying case is solid and actually quite useful. It has a shoulder strap and a loop for stringing it through a belt and will take both lens and lens hood. The lens hood is, well, a lens hood.
In terms of design it's not the newest, and there may be room for improvement, but it's difficult to find another (Canon fit) zoom lens of this range and quality even at around £1000.