Incidentally, I can confirm the Bell Formula MIPS (possibly their best value) works as claimed, within the limits of my single experience: I was smacked on the head by a railroad crossing arm*.
The MIPS system did what it was supposed to do, and deflected the blow while the harness remained securely in place on my noggin. Nary a scuff to the polycarbonate shell, no noticeable impact to my head. While I'm pretty hard-headed, that direct blow without a helmet would have hurt, probably with a scalp injury at least.
*(Long boring story -- I was tailgunner for a casual group ride and stopped about 10 feet before the tracks when I heard and saw the caution signals. While I was looking around to check on the group and the distance of the train, I neglected to notice I was standing directly under the downswing of the crossing arm. The train was so far away and apparently not even moving I was surprised at the alarm and simply didn't notice I was stopped in the path of the crossing arm.)