X-Forwarded-For: Multiple IP's Chained | Need Original IP | iRules and HTTP Profile?

This is a case of multiple XFF headers and you can't really know which contains the end-user address unless you know the addresses of the intermediate systems to filter them out. To log all the addresses locally, you can do:

when HTTP_REQUEST {
    if { [HTTP::header values "X-Forwarded-For"] ne "" } {
        log local0. "[HTTP::header values \"X-Forwarded-For\"]"
    }
}

You will need to make some changes for remote logging.