Conditional tag to show content in custom rss feed?

Okay, that was quite easy. So, if your custom feed URL (non-pretty) is http://example.com/?feed=wpse63760_custom_feed, then the conditional tag would be used as such:

if ( is_feed( $feeds="wpse63760_custom_feed" ) ) {
   // So what's the conditional content?
} else {
   // This is shown everywhere else
}