Thanks for the insight.  In answer to your question on what I would use
it for, it would be mostly wildlife in general and birds in particular.
I presently use a point and shoot type Nikon 8700 that has served me
well for several years, but now that I have recently retired, I plan to
upgrade to a DSLR system and spend a little more time at it.  It will
however always be a hobby for personal pleasure, and not a profession.  
An activity that I can enjoy and that will get me outside.  

I did do a lot reading on the web first and thought the 100-400 zoom
best met my needs and budget, but after reading your post, I went back
and did even more research.  You are right, the 300 mm F4, does look
like it produces better images, but what really convinced me that the
100-400 was not the best choice was a statement I ran across in one of
the reviews that stated that the zoom was not all that useful if you
are going to shoot most of the time at the full 400mm. That reminded me
that for what I have been doing, 99% of the time, there was never
enough reach.  I don't think this will be any different. Even using the
300mm with the 1.4 TC at 420mm, I will probably be always wishing I had
more, not less.  I had blinders on and was convinced that I wanted a
zoom, so that is what I had looked at, and was giving up picture
quality in the process.  I still plan on a zoom for a general walk
around lens and casual shooting, but that is another story.   Thanks
again.

In regard to the other posters that answered my original question with
helpful responses, or supported Roger's assessment, I appreciate that
you took the time to answer.  I have read them all.  

When you spend a lot of time on the web reading, you do come across
posts etc. where people have had trouble with a particular lens, and in
at least one case, they stated they had used two samples of a given
lens at various times and one they had been happy with, and one they
had not.  You suspect such instances are few and far between but you
don't know for sure.  Your comments have helped reassure me that they
are indeed isolated instances, and the response is generally good by
the manufacturer when they do occur.  Thanks for your patience.

An unnecessary comment, Al, and frankly rather thoughtless.
He's asking a legitimate question about a fairly expensive lens.

Your silly comment aside... I've owned two of these lenses (first was
stolen, and replaced by another).
Both were excellent, though I wished for larger a larger aperture.  In the
end, I tended to leave the 100-400 at home, taking the 70-200 2.8 IS
instead.

Tony Polson has to add his own FUD as well:

I have it on good authority that when a person goes into Canon,
complaining about the AF performance of their camera, the desk people
are instructed to act sympathetic, collect the camera and ensure the
customer their issues are being put at the front of the repair queue.
The Proper Lens Active Camera Enhancement Body Operation is then
performed on the camera:  it sits on a shelf for a few days.  The
customer is then called and told the problem is fixed.  Interestingly
enough, they never come back for repair...

A proper SLR system does not have to be "matched".

While I disagree with the rest of your post, no truer words than above
have ever been stated about high end gear (photography and otherwise).

Cheers,
Alan

Canon and other mfgs who use the 3:2 ratio have a built-in bad lens
fix:  The ability to crop off the sides of the image if the quality is
poor.  The top and bottom don't matter since the lens image circle (and
edge quality) fall well outside them.  If Canon had a more square
sensor format that made use of more of the lens image circle, you'd
hear many more quality complaints, especially from users of the FF
sensor cameras.
That concerns image quality.  However, Canon's AF system is (from all
reports) as good as currently exists.