The only negative reports I've heard on the MiFlex hoses are from a diver who uses scooters a lot, and had problems with the more flexible MiFlex hose flapping around too much when scootering. It's a thread here on SB, I'm just too lazy to search for it right now.
Not being one to use scooters myself, I have been converting my hoses over to MiFlex after using one for the first time when I built my pony bailout. This next week I'm planning to replace my HP hose with MiFlex as well, if my LDS can adapt the Sherwood Wisdom computer's quick-release connector to the standard HP MiFlex (they claim they can).
Now that I'm running my reg off a long-ish MiFlex hose routed underarm (and with a 90-degree adaptor), I have very free movement and never feel tension or pressure on my mouth the way I sometimes did with the stiff rubber hose routed over-shoulder, even with the Atomic ball mount (which I removed when converting to MiFlex).
I still have one rubber hose, an LP inflator hose for my i3 inflator, but there's actually an advantage to keeping that a stiff rubber hose, because it's routed down along the back of the BC, which is harder to do with a more flexible line (especially if the tank is already strapped in). Also, once it's connected, there's really no need for it to flex much and it doesn't affect my movement, so I haven't yet seen any advantage to converting that one to MiFlex.
>*< Fritz