A bit of background: It gets super hot where I ride, and in 110-degree heat you need lots of vents. So I got a Bell Super3 MIPS mtb helmet to replace my mostly solid-shell urban helmet. The increased ventilation was a miracle. Riding at speed, seeing others' nasty experiences, I bought the chin guard in late November 2017 with the idea being I would only use it in circumstances I felt where the increased danger warranted it. So now it was a Super 3R.
Soon after, I decided it was too much of a PITA to put the guard on and take it off, so I left it on and lived with the goofy look.
Two weeks later, I was t-boned by an inattentive motorist (in the bike lane, 15 mph accelerating slowly from a complete stop, driver was stopped in a parking lot ramp looking at me, but it turns out she was looking thru me despite three Niterider headlights).
So, I fly up thru the air Superman style, which was kinda neat, but on the downward arc at that speed I knew I was in deep s*it. I landed on my head and kept skidding on my head as my body arc'd over to eventually slam onto the pavement and flop around to a stop. During that body arc, while my head was 'anchored' to the ground, I felt MIPS doing its job in real time as my head and body turned but the helmet did not.
During the flopping-and-flailing phases as I came to a boneless stop, my right eye vision got all fuzzy. Turns out it wasn't my eye. It was the eyeglass lens. As my face bounced around inside the helmet, at some point I came very VERY close to the pavement with my eye socket. And my eyeglasses. Note the lens is scratched but the frames are pretty much not. That was the chin guard saving me from reconstructive surgery at the least.
And the helmet was... well, smashed. The initial impact and slide on the top of my head shattered it, and you can see the scrape marks. the outer shell creased but remained intact. I had two cuts on my noggin from plastic fragments but otherwise my head and neck were uninjured. However, I pretty much sprained *everything* and split some ribs, and trashed my wrists. Still, I got off easy.
I bought exactly the same helmet as a replacement. Recently I decided the Bell Super3 DH was enough of a tech improvement to buy one. Instead of the thin plastic MIPS membrane, the new one has an entire secondary inner super duper styrofoam liner that does the same job, but provides an added layer of protection. Its a little bigger so the Kazoo effect is there to a degree, but its a worthwhile upgrade.