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A circular polarizer is just a linear polarizer combined with a quarter
wave retarder, so the polarizing effect of a circular polarizer is the
same as the linear polarizer from which the circular polarizer is made,
including effects when the polarizer is rotated.

The only real difference, all other things being equal, is that a
circular polarizer tends to be a bit more expensive than a linear
polarizer.

A circular polarizer can always be substituted for a linear polarizer,
but not vice versa.

A polarizer typically results in a bit of color shift.

What can and does differ from polarizer to polarizer is efficiency of
polarization, optical quality, and amount of color shift -- filters are
NOT all the same. Putting a cheap filter on the end of a good prime lens
is a good (actually bad) way to degrade the image.

Best (and most expensive) filters are B+W/Schneider and Heliopan.
Good filters are Hoya, Kenko (my own favorite), and Tiffen.

Good filter sources include:
* http://www.bhphotovideo.com/
* http://www.2filter.com/
* http://www.filterhouse.com/
* http://www.photofilter.com/

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Best regards,
John Navas <http:/navasgroup.com>