@Matt, unfortunately, the c-ip value in IIS is the IP of the client side connection to the LTM, it's not a header value that can be changed with an iRule. You can setup your LTM configs to pass through the origin address but in some situations that isn't possible when NAT is required. The XFF Filter, looks into the HTTP request and if a X-Forwarded-For header is found, it injects that value to the log during a log operation.
As for stability, we use in on DevCentral and have many users who have deployed it and I haven't heard any issues. The source is available if you have concerns...
-Joe