I have been extremely happy with the ecometer. I paid around 80 dollars for it including shipping but have since found out it can be found for cheaper if you shop around.
The unit is compact in size yet very easy to read. It includes a digital tach, speedo, avg. mpg and instantaneous mpg. It also has an "ecograph" that is a bar graph that goes around the circumference of the face to give a relative indicator of efficiency in all modes of operation.
It plugs into the obd II port of the vehicle then mounts with included 3M industrial "velcro" and your done. No permanent alteration to the vehicle is required. My ecometer's avg. mpg has always (except for the first tank) been within .9 mpg of the "calculate at the pump method" which leads me to believe it is actually the more accurate indicator of actual fuel economy.
This unit does not provide as much engine data or read/clear obd II codes like the Scan Gauge II but if you don't need or want that capability this is a great working instantaneous mpg gauge and tach that updates 2x per second. If mine were to fail or was stolen I would buy another pronto-would not want to be without it now.
The ecometer has helped me avg in excess of 40 mpg lifetime so far in my Yaris despite mostly in town/rural driving with the majority of my trips being 10 miles or less.
The following forum thread is rather long but if you care to skim through it contains quite a lot of information and mounting ideas for the ecometer.
http://www.yarisworld.com/forums/showthread.php?t=18650
BTW I nor any family work for Auto Meter, lol! The ecometer is made in the USA also, which I find interesting. I have joked about how I never would have believed as a teenager that one day I would own a Japanese car with American made electronics!