just curious, but which cases? my two from Spigen didn't quite fit. the buttons on either side didn't reach. Incipio's current 6 Plus cases won't fit as well, and they even suggest waiting for their 's' versions. but their cases are pretty snug.
My Spigen fits but did not try long so while I saw no problems, I am using/testing a tech21 Impactoogy classic check that i had on my 6 Plus, it fits well.