An easy way to test this and get a rough estimate is to check file sizes with compression turned off and on in your control panel.
This page (before this post was made)
with compression: 3755 bytes / without compression: 21029
the forum summary (ubbthreads.php)
with compression: 4423 bytes / without compression: 30248
You get roughly a 80-85% savings on the html part of your forum, tho none on your graphics - this means the % will go down depending on how much of your site loads new graphics on new pages. The graphics do get saved in the user's cache once they are downloaded the first time, so savings go up with repeat customers []/forum/images/icons/smile.gif[/]
There is a hit on the server's cpu fpr doing the compression, so a busy site on a less-than-stellar server might need to have compression turned off.
