The Scangauge is a little black box that is loaded with features. It works to read your trouble codes, it acts as a digital readout of your gauges (speedometer, RPM's, coolant temperature, and more!) and it can also be used to keep track of your mileage on trips.
Once my truck got finished up with it's "conversion" (summer-fall 04 I repainted, lifted, tired, wheeled, regeared, re-interiorized, etc.) I had the opportunity to use the Scangauge on a trip out to Las Vegas for the SEMA Show, and this was the first chance I really had to use the Scangauge. Even though the speedometer was slightly off (hadn't been corrected for larger tires yet) I was able to track trip time and mileage with the unit. I compared it to notes I was taking on my own from the odometer, and the mileage was very accurate (unfortunately - since I was only getting about 13 mpg thanks to the larger tires and underpowered gears).
I also had a chance to use the Scangauge to read out the codes when a friends air filter started letting too much junk through, causing an engine code. This was easy, it just involved hooking it up to the OBD-2 port, checking the code, and then resetting the light. The only thing I didn't like about this, and its true of almost all code readers, is that it didn't describe the code, it simply gives the code. But thats ok, because the internet has a plethora of information available about engine codes. A quick check of codes on the internet reveals the code info and often troubleshooting tips.
You have several choices as to where to mount it, depending on how creative you would like to get and how visible you want it to be. Because of current aftermarket equipment already mounted in the truck, I mounted it to the underside of my Cobra CB. It can also be mounted on top of the dash, under the dash, in center consoles, or you can get creative!
Overall, I really like this unit and think its definately worth the cost (about $100). Linear Logic (the maker of the scangauge) even offers a 30-day money-back guarantee so if you don't like it, just send it back, but I'm sure you won't.
Sources:
Scangauge
www.scangauge.com
scangauge@linear-logic.com