I have a POC Cortex Flow DH helmet with MIPS. I got it because it fit me well, was comfortable and light - carbon shell - and on sale. I've read up on the MIPS stuff - it uses slip layers in the helmet liner that are supposed to reduce the rotational energy delivered to your brain which makes it "wobble" in your skull. I've had a couple of small spills on it and I can't say if I noticed anything different, but it seems to do its job just fine.
There is some dispute as to how effective MIPS really is. Ultimately, however, there are some impacts that your helmet can't protect you from (like a neck-spinal injury). So long as you take reasonable measures (like wearing a properly fitted helmet and wearing a neck brace for DH/Park), you are probably doing as much as can be reasonably expected.
PS Found this article
Surprised so few people ride with a mouth guard
Many people find mouth guards annoying if they have to be worn for any prolonged length of time.