I just posted something on this in another thread on helmet selection, and I will reiterate here.
Note: I am by no means an expert on helmets!
I just bought a new non-MIPS helmet.
I did some research online, and this article by this group that does a lot of research on helmets sums it up pretty well for me:
MIPS and Sliding Resistance of Bicycle Helmets
Basically they say MIPS has not really been proven to work, it might work. It might not.
The tests MIPS do involves dummy rubber heads that fit very tightly around the helmet, so the extra slip plane really makes a difference there. But a real head already has some 'slippage' between the head and the helmet (hair, scalp, air space,...), so the added benefit of the MIPS slip plane is not certain. This makes sense to me, as pushing any helmet on your head will allow it to shift a couple of mm.
Anyway, not any of the helmets I liked or could afford had MIPS, so I went for the other option. I'm not saying it is not a good technology, but not significant enough for me to alter my choice.
Maybe an independent testing body will do independent tests on MIPS (Bell and Giro own a stake in MIPS apparently, so their claims are hardly unbiased).
Maybe it will become the standard in helmets in a couple of years, who knows. It seems like it is too early to say one way or the other.