Just in case anyone thinks I'm being unfair on Autopano, I should point out that the above comparisons were largely for auto mode and default capability.
I would say that CS3 Photomerge is the best stitching program that I've come across for fully automatic, load and click okay, stitches.
The problems arise when the fully automatic (or default) modes don't work. What is the potential of the program then.
There's always some degree of learning curve. To fairly compare the full capabilities of different stitching programs requires that one become fully conversant with those programs.
I downloaded the trial version of PTGui and found that that program also appeared to be incapable of stitching the last two images of the above 3 image project. It couldn't find any control points. That's very odd, I thought.
I went back to Autopano and after messing around with the 'settings' and basically pulling out all the stops, increasing the number of key points per image pair to the maximum of 200, forcing every picture to be in the same panorama, setting 'find control points everywhere' etc etc, I was able to get what appeared to be a good stitch of the 3 images. However, on close examination at 100% magnification, I saw that the figures in the foreground were terribly blurred and mixed-up.
I went back to the settings and changed the default bilinear interpolation mode to bicubic and the default multiblend to 'smartblend', then tried again.
This fixed the problem and the resulting stitch is actually better than the CS3 stitch. The horizon is straighter, which means less work with free transform and warp or distort. Even the sky joins are slightly smoother. There's a hint of darkening at the joins which is not there in the Autopano stitch.
However, these same settings which have produced this marvelous 3-image stitch, better than the CS3 stitch, have not done as good a job with another set of 4 images (the first 4 in a series of 60 or so). But neither has CS3. They both have their faults. The CS3 stitch has a smother sky but a curved horizon. The Autopano stitch has a major problem in the sky but at least has a straight horizon.
PTGui produced a result similar to Autopano. To find out how to improve upon that I'd have to become fully familiar with the program. I don't need to because Panavue's Image Assembler, with which I'm reasonably au fait, can do do a perfect job, but not in auto mode of course.