The Cardboard is amazing! Simple, cheap, and guess what? The effect is really good! I almost feel I regretted to spent $500 on the Oculus rift.
The result is actually pretty good! The 3d effect is very nice and real.
I guess there will soon some cheap gadget to buy with better quality. Right now you can build it on your own with some paper boards. Google website has the instruction and PDF design template for you to cut paper. https://gweb-cardboard.appspot.com
Google even setup a booth to let you compare the Oculus Rift side by side to show Facebook bought something a bit too expensive.
Guess which is better? I think it's Google Cardboard. And I randomly asked a few people around, most agree with me. (The model google used to compare is first generation Oculus, it has lower resolution than a typical android screen, so not sure how new Oculus compares)