Since Michael chose not to take a position and I did purchase the XF300 not long thereafter, perhaps I should answer my own question. There may be others with similar concerns.
First and foremost, I am happy with the XF300 autofocus (and assisted manual focus). Like all autofocus mechanisms, it is not perfect; but it is extremely useful. It has made it possible for me to get unpredictable, high-action shots easily that would have been virtually impossible with my old Panasonic HMC150.
I should note that the single most important improvement relative to focus on the XF300 has nothing to do with the automatic mechanism. It is the high resolution viewfinder and LCD screen. If you are shooting 1920 x 1080 (2.07 Mpixels), then it is almost impossible to spot focus errors while you are shooting with the HMC150. The viewfinder and LCD screen lack the necessary resolution. Of course, you can zoom the lens and press the “focus assist” button to magnify the image, but that takes you away from the action. Your shot is lost even if the focus was good. The XF300 has a viewfinder and LCD screen which provide a fairly good indication of focus while you are shooting. (The Canon is even better than the venerable Sony PMW-EX1R.) As a practical matter, I find little need to zoom the lens and/or use the “magnify” button. As I shoot, I can see if the autofocus is losing the target. It does happen on a few occasions, but I know right away that I need to do something to get a better shot. With a little experience, one learns where the focus-sensors are located within the frame and can follow the action with very few problems. I may regret saying this – knock on wood – but I have yet to come back from a shoot where something I cared about was out of focus; and I failed to re-shoot a good version. In other words, the screen resolution makes imperfect autofocus viable and useful.
As for the quality of the instant AF (using the special sensor at the side of the lens), its speed and accuracy dwarf the HMC150. On a few occasions (and I do mean only a few), it will “lock” on to the wrong subject. But if you pay attention to the viewfinder, you can see the error and reframe the image so the autofocus corrects. Take a look at Canon’s special “edge monitor” display. It shows three regions within the frame that the canon engineers regard as “important”. As noted above, a little experience goes a long way toward keeping the active subject in the camera’s “hot-spots”.
In all honesty, I have not made much use of assisted manual focus since my luck with autofocus has been so good. Based on my limited experience, I found no need for the “magnify” button. I just hit the “push AF” button. If the distance readout on the screen seems reasonable given the depth-of-field, then I am done. If I am uncertain and I really need to nail the focus with absolute precision, I will zoom the lens, re-hit the “push AF” button, and then zoom back. It is pretty quick and relatively painless (given that we are talking about manual focus). Of course, if your video subjects only have predictable movements (e.g., scripted action with professional actors who know how to hit their marks), then you can use manual focus (assisted or not) all the time. If you are lucky, you will have an assistant cameraperson to pull focus for you during action scenes. That certainly is the professional way to handle focus. But if you are a one-man band, then autofocus can be a very useful tool.
P.S. The HMC150 is a fine camera that I very much enjoyed using. The XF300 has some advantage as you would expect from a camera at twice the price (and with newer technology).
