I purchased one of these the same day that gpracer1 posted the link. The $ was the deciding factor and wouldn't hurt my wallet to bad.
So far I really like it. It was simple to install and setup. I haven't spent to much time customizing it yet because I really wanted to get a feel for the default operation and then go from there.
I am getting better mileage than I thought, about 16.5mpg overall. I in recent weeks I have been towing my boat a lot and that has killed the overall average. I'm doing about 12.5 with the boat (4500lbs) at 60-65mph.
My speedo does match the scan gauge, and it's nice to have a better fuel gauge (pending you update the unit on every fill-up). I have a VW CC that's got a trip-computer system like this and I do prefer the digital readout of the Scan Guage over the analog in the Tahoe.
For the money, this is a pretty slick system and the website is full of useful information.