$Is anybody using this lens?

  Yes, there are people using this lens.  Google will find some of them
for you.  Find the FAQ at birdsasart.com, for instance (and you'll
probably have to find it via Google, as the link to it off the home
page of that site seems to have disappeared some time ago), and
you'll see it labelled the world's best lens for shooting birds in
flight.

$                   400 is about the minimum reach I could use and will
$have to use a 1.4 TC on occasion, so I'm leaning heavily toward a prime.

  I hope you're aware that, on most EOS bodies, the effective
f/8 of the 400/5.6 with a 1.4x TC turns it into a manual-focus lens.
Only those with the 45-point AF system (EOS 3 and 1V on the film
side; 1D family on the digital side) retain AF, and even at that
only with the centre AF point.

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    Say hi to my cat -- http://www.stevedunn.ca/photos/toby/

I use a Sigma f/2.8, 120 - 300 lens for long shots with a 2X
teleconverter. This runs out to 600 mm at the saem aperture as the 400
Canon. On a 1.6 crop factor camera it gives the field of view of a 800
mm lens but the depth of field of a 600.

There are two drawbacks in this lens.  1 is the dinner plate size
filters you will find horribly expensive and the in low light it hunts
for focus. Better to just go manual. Otherwise, it is a really first
class lens. Some slight purple fringing at extreme magnification but
nothing that can't be dealt with fairly easily.

The Canon lens is supposed to work with a teleconverter but the thing
will go out of auto focus mode when you use one. The Canon is about
twice the cost of the Sigma and no better at resolving detail.