I've got one on my Ford Explorer, but it is set such that it turns the engine off after 5 or 10 minutes (selected by a switch setting). And actually, I've tested it and the Explorer would have a positive charge to the battery when it was idling with the headlights off.

I liked it on the Explorer, as before I retired, I would drive 3/4 mile to the freeway and then everyone would be driving 75 mph. The extra few minutes of an early start before I went out the door gave it a little more time to warm-up before hitting those 75 mph speeds. While I understand the general instructions to just start the vehicle and drive easy until it warms up, in my case that wasn't really practical. And jumping to a 75 mph cruise two minutes after starting just didn't seem to be a great idea.