differential narrowing jig

Whats your thoughts on differential narrowing jigs and rulers? I see that some cost upwards of 650 for the Mittler Bros setup and was wondering from someone who has the tools already...what you have...what you wish you had....and what is working for you. I am willing to drop cash on good tools but am hoping to glean some knowledge before diving in. I have a couple projects that need diff massaging. Might as well learn how to do it.

Thanks FABO!