CHAR vs VARCHAR

I browsed the FAQ and mailing list briefly to try to garner information
about the ups and downs of different character based datatypes.  I did not
see any good document on preference or when to use one over the other.

http://postgresql.readysetnet.com/docs/faq-english.html#4.15 noted that
some compression happens.

http://postgresql.readysetnet.com/users-lounge/docs/7.0/user/datatype1066.htm
gives a good description and notes that text is best choice, but it not
being SQL compliant is an issue for most.

The question is how much of speed difference is there between CHAR and
VARCHAR when doing searches etc?  or are there any good rules others use
to determine when CHAR is better to use than VARCHAR or vice versa?

Derek Neighbors
GNU Enterprise
http://www.gnue.org
derek@gnue.org