I've been using mod_gzip on my weblog server to try to save bandwidth. Today I crunched some numbers and learned that gzip encoding only works for about one third of the web requests for HTML that I get. When it does work, it compresses to about 30% of the original size.
Turns out that while most user browsers support gzip encoding, most spiders don't. GoogleGuy says this may be because servers don't reliably serve gzip. I could believe that given the contortions I had to go through.
RSS aggregators are mostly good about supporting gzip. They are good about handling 304 Not Modified, too. Good thing; RSS polling is such a huge source of traffic.