Folding the mirror and dropping the plug in place is quick & easy - I can do it on the go. The blank has a "pin" that drops into a hole drilled in the exposed hinge. Lower window, fold mirror, drop in plug. No muss, no fuss.
This is the only test I've done with the Insight so far, so it's a single speed vs. mpg data point, but the Firefly is good for ~65 mpg (US) at 80 km/h (50 mph).
Lean burn 1.0L Honda, lightly modified: ~95 mpg (US) @ 80 km/h (50 mph)
Non-lean burn 1.0L Suzuki, moderately modified: ~65 mpg (US) @ 80 km/h (50 mph)
Lean burn wins, hands down.