I recently faced this same dilemma.  I had lost a polarizer,
and was about to leave on a photo trip, so I went by a
camera store and asked for a quality circular polarizer.
They guy pulled out one (forget the brand) that was over
$200.  I asked for a good consumer model.  I then compared
the two (using two of each filter):

1) for each side of the filter, reflect sunlight onto a
distant wall (preferably outside).  The projected reflection
should look like a perfect ellipse with no distortion.

2) look through the filter held at arm's length, and tilt
and rotate it.  Look for any distortions.  If you can get
a second person to hold it well way from you look through
the filter that way.  Any deviations from plane parallel
with show stronger the further you away you look.

3) take two polarizers and stack them, then rotate them
while looking at a light source (e.g. light bulb or a diffuse
reflection, like from white paper).
Good circular polarizers will show NO extinction as you rotate
them.  Be sure to test both orientations (e.g. like stacked
filters and front to front/rear to rear).  Cheap filters
may give near complete extinction.  If the transition is
smooth with no variations in transmission at different locations
on the filter, it may still work (but the filter is not a great
circular polarizer).

4) if you can look at the daytime clear blue sky, try the
polarizer out on the sky (without a camera).  Hold the
filter up and look through it while looking at a patch
of sky 90 degrees from the sun.  The filter transmission should
be smooth at all locations on the filter as you rotate the filter.

I use Hoya filters and find them very good and reasonably priced.

I did find a filter that failed to pass the above tests one time
many years ago.  It showed a blurry image when looking through the
filter with the filter at about 10 feet.

Roger
Photos at: http://www.clarkvision.com

 While there are differences in the type of glass and what the rings are
made of, the largest difference is whether the elements in the polarizer
(there are two, in between which is sandwiched a thin film) are uncoated,
single-coated, or multi-coated.  Google and read on lens coatings to see
what effect they have on optical performance.

steve

The circular polarizer is required for autofocus to work. Something to
do with the way the autofocus sensors work means they won't work with
the linear polarizer.