I had been looking at getting the OR DK2 for a while, but what was putting me off is that I have pretty bad sight in my right eye. I don't watch 3D movies as they quickly makes my eyes ache, followed by a headache, and I didn't want to spend the money on the DK2 if it was going to be the same.
So, I ordered Cardboard, stuck my HTC One M8 in and was pretty impressed with what it could do, and the fact that I was actually seeing proper 3D without the eye pain. After a week of playing with it I ordered the DK2 and have had no problems with that either.
It probably will vary from person to person, depending on any eye problems that you have, but Cardboard is a pretty cheap way to test what results you can expect before you waste your money. And if don't mind spending some time setting it up it will even run Elite Dangerous