I also have a ScanGuage II. It is velcroed to the top of the steering column just where yours is shown in the photo. I have the cord wrapped around the column several times just to get it out of the way. It is saving enough money on gas to pay for itself.
I would highly recommend it if your vehicle does not have transmission temperature in the displays someplace. Having this information while you tow, especially in the mountains or in very hot weather, might save you the cost of a transmission replacement. See The Secret to Extended Automatic Transmission Life - WAYNE'S TRANSMISSIONS
for more info. There is a nice chart on heat vs transmission life at Transmission temperature/failure chart
Most folks over on the fuel economy forums claim you can get an quick increase in your fuel mileage by getting a good gauge and paying attention to what it is telling you. Stories of people getting 5% to 15% better miles per gallon within a week or two are common. (It worked for me). Set two of the readings to AVG and MPG. AVG is the average fuel economy for the trip, MPG the instant fuel economy. (Some of the more recent cars show these in their displays somewhere.) Your goal is to have the AVG reading as high as possible. Watch the MPG as you drive and it will show you how your driving style at the moment affects your mileage. Try it on a trip you do all the time like your commute and you will see what you are learning to save. For advice search "hypermiling tips". What you will find ranges from easy to stuff that takes a little more practice. (There is a little of the totally stupid in there too). The page at 100+ Hypermiling / ecodriving tips & tactics for better mpg - EcoModder.com
is long but very comprehensive.
A good gauge has the added advantage of being able to read trouble codes your vehicle's computer stores when it detects a problem. If you are trying to trouble-shoot a problem being able to read and clear those codes is helpful. Having a tool that will tell you if the Check Engine light is a big problem that needs to be dealt with quickly or a smaller problem that can wait a little is also convenient. It will also give you a check on what your mechanic is telling you if you don't DIY on your vehicle. Your favorite search engine will get you lists of codes and what they mean.