My specific goals were to investigate some aspects of the lens which weren't really issues at the cricket match: the ergonomics of the lens on a monopod; how it handles in situations where you want to zoom frequently and rapidly; and the AF speed on moving targets.
Sheepdog trials photography practicalities
Here is an overview of the course from the spectators' viewing area. It's HUGE - about 400m long (left to right in the photo) and 250m wide. Much bigger than the fields used for any ball sports, even polo. Clearly the action at the far end of the field is going to require a very long lens indeed.
Camera phone panoramic photo
For those who haven't seen "One Man And His Dog" on TV, here's a quick overview of what happens. The shepherd stands at his station on the right. The dog runs from there down the far side of the course ("the outrun") and sneaks up behind the sheep on the left, aiming to get them moving ("the lift") with the minimum of fuss. The dog brings the sheep back to the shepherd ("the fetch"), ideally in a straight line, passing through the pair of gates on the way. The dog then rounds the shepherd and takes the sheep out to one side of the course, across the course through two more sets of gates ("the drive"), and back to the shepherd. Then the dog has to temporarily separate two sheep from the flock of five ("the shed") before the dog and shepherd take them to the pen where they are corralled ("the pen").
(Or rather that's what ought to happen. On TV, where the competitors are amongst the best in the country, it usually does. In a local event like this, things are much more unpredictable!)
Obviously a lot of the action happens a long way away. Using Google Maps I estimated that the sheep release point was about 300m from me, and the shepherd's station about 150m. With those sorts of distances to contend with, even 400mm x 1.4 = 560mm focal length isn't going to be enough reach; if I were doing this seriously I'd probably use the Canon EF 800mm f/5.6 L IS USM. But during the "drive" phase, the dog and sheep would be coming more or less straight towards me - probably quite quickly and erratically - before turning to go through the first pair of drive gates, so hopefully that would present some interesting opportunities.
As before, all images below are straight out of the camera - no processing, and no cropping.
I have to say that the lens is an absolute joy to use on a monopod. It's perfectly balanced. The zoom action is very light and fast. Holding the lens with my left hand on the zoom ring, so that I could easily change the framing of the picture, was very easy indeed. For example, frames  and  below were taken less than two seconds apart. In that time the dog dashed towards me, stopped, turned, and started moving again; I went from maximum zoom to nearly minimum zoom, re-framed, re-acquired focus, and took the shot. It was surprisingly easy.
One aspect of the lens which is very easy to overlook is that, when you're using the built-in Extender to increase your zoom range, the lens is no longer a fixed-aperture zoom. It's effectively a sort of 200-560mm f/4-5.6 zoom. This means that you have to be wary when using manual exposure. For example, with constant strong sunlight under clear blue skies, I could just dial in something like 1/2500th at f/4 and leave the camera set like that all day. That would avoid any metering issues with white-ish sheep and black-and-white dogs. But as soon as you switch in the Extender, 1/2500th at f/4 becomes 1/2500th at f/5.6 and everything is under-exposed by 1 stop. Of course there are workarounds: use f/5.6 all day, or tweak the shutter speed or EC whenever you use the Extender switch, or use aperture priority and deal with any vagaries of metering. I chose the latter approach. As you'll see from the images below, there's a natural tendency for images of black dogs to be a bit over-exposed, but it's easily dealt with.
The AF didn't struggle to keep up at all. As with all of Canon's big white lenses, it seems to be essentially instantaneous. Of course, an air show or motorsport event would tax it a bit more, but I'm confident that it would cope quite easily. A border collie running flat out is faster than most things you'll see in the sports and wildlife arena, so it's obvious there won't be any problems there. I sometimes had difficulty maintaining the focus lock on a running dog, but that's because I was using my trusty old 40D rather than a camera with a more capable AF system. The difficulty was tracking the target as it moved from one AF point to another, and that's the camera' s job, not the lens's.
Overall, I'm very impressed. I certainly couldn't justify £12,000 on a lens for personal use, but if I were a serious wildlife photographer I would definitely be putting it on my Christmas list and trying to persuade Santa that I'd been a good boy. Or I'd be hiring it as and when needed from the UK's #1 lens hire business.
 f = 290mm, 1/2000th at f/4
 f=560mm, 1/1000th at f/5.6
 f = 560mm, 1/800th at f/5.6
 f = 560mm, 1/1000th at f/5.6
 f = 200mm, 1/3200th at f/4
 f = 560mm, 1/1250th at f/5.6
 f = 297mm, 1/1250th at f/5.6
 f = 200mm, 1/3200th at f/4
 f = 250mm, 1/3200th at f/4
 f = 400mm, 1/2500th at f/4
 f = 560mm, 1/500th at f/5.6