It's useful. I received my Eco-Route HD in the mail from ebay during the week my check engine light came on. It is actually the Kenwood version of the Garmin but all the same. Anyways, I accidentally did not tighten my gas cap enough and got the dreaded check engine light. Well I clear the code with the Eco-Route and BAM....never had a check engine light ever since.
When I do check my Garmin gauges, I usually like looking at the coolant temperature gauge to know if my engine is warm enough to drive normally. I also like looking at the battery voltage gauge since I have alot of electrical accessories.
I also like to look at the leaf to try to drive efficiently. It turns red-yellow-green which gives a score of 0-99. BUT lately I have been looking at the real time MPG when I place the garmin screen in detail mode for my particular model in addition to looking at the AVG mpg on the Nissan trip computer. ( not as accurate I know but to get an idea)
If your on the fence about buying this, I would get it. All in all, I found it useful. Code clearing, saving gas, and gauges.
ps- It would probably be cheaper to get the iPhone

or android version.