The site I moderate on will not turn this feature on as it is for Spammers.
A snippet quote from the admin"Yup, were not turning on RSS as it will allow our posts to be fed to other sites...."
That wouldn't really be spam. You're talking about other web sites stealing your content. RSS is a horrible way to steal content since it really only shows the title and a short description of the item (the specifications call for less than 256 characters). RSS does NOT allow anyone to feed content back to the site either - only read it. "Spamming" is when someone posts garbage ads on your site and is technically impossible with RSS. I believe you *can* make the feeds private if you're worried about people accessing them without permission.
As for performance, since the information is pure text in a relatively compact format (usually 15 items or less at 256 characters) there's much less of a load on the server than actually having people read all those threads individually. On a "normal" site, the XML file is stored by the user for a predetermined amount of time. This means that no matter how many times they view the RSS feed it only makes requests to the real web server when it determines the file is "stale". This is because RSS is mainly used for news type feeds which may only get updated a few times per day. On a forum, the feed is updated much more often but I'd bet there's still a small time delay.
Also, MyYahoo is very simple to setup for reading RSS. However, it doesn't seem to update as often as it should. I haven't timed Google yet.
You're welcome Red!
Troy