well, there is no MPG payout. don't get me wrong, your MPG will improve.
the local news station around here does this thing called "DEAL OR DUD" where they review products (usually infomertial crap) and tell if it is a deal or a dud.
the reviewed the scangauge and the conclusion was that even though it didn't actually increase your efficiency (the product itself) it made you more aware of your driving which in turn probably would increase your MPG.
it is also a wonderful tool for experimentation as well. their web site shows all the things it can show and the X-gauge expands on that even farther. if you know computer code (and your cars code) you can even write your own X-gauge codes (never done it but some on here have played with it).
check them out at www.scangauge.com there are several places to get them from. I would definitely shop around. check other forums as well as jcwhitney.com and thinkgeek.com you may be able to find it cheap. you can also check ebay and amazon for a used one.
there was a member here that was getting rid of his. he has since moved on but I will send him an email to see if he still has it and what he would want for it.
Be the change you wish to see in the world