We all drove it....since it was only one of a few Chevy had, it was not ours to keep. That was last September I think (Note green grass, and no snow drifts!)
Electric up to 35mph, then the 6.0 kicks in. MPG is increased with mild braking (which is the best way to recharge the battery, with its regenerative brakes), and slow easy starts.
I think the latest rating was 20/22 mpg on avg.... We'll see soon - I have two orders on standby ready to be entered into the system!