It's cool-ish, maybe not worth the $20 I dropped on it, but it amused the family for a weekend.
There are a few issues with Cardboard that are annoying:
1) The rubber band to secure the phone flat out sucks. It gets in the way of the right side of the screen and it's a pain to slip the S6 under the rubber band just so.
2) There's no way (that I could find) to control the Cardboard app interface without opening up the front repeatedly. After one weekend, the velcro holding it down is on the verge of being shot. Then you tap the screen but the darn rubber band keeps getting in your way.
3) The apps are fun... for about 15 minutes. Nothing substantial to keep me going back, although Photospheres are hella cool looking with VR.
4) As the name states, it's made of cardboard. I used it for one weekend and it looks like it went through the wringer. I didn't even use it that much. It collecting dust now.
5) Believe it or not, even with 577ppi, there is still a very noticeable screen door effect.
BTW, the S6 gets really hot, but it is what it is.