It may not be your high end gaming VRE headset, but with just a few pieces of cardboard and a phone (Nexus 5 should be but you can adapt), you'll get a fully working VR headset with audio, gyroscope, 3D, etc. That little metal ring on the side is actually a trigger (using the Nexus 5 magnetic sensor).
Google Cardboard is more of a standard than a single product. It started with a cardboard box with glass lenses and a slot to hold your phone, but now there are many different hardware solutions that are all just as capable as the next.
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Pros
Pro Very low barrier to entry
Google Cardboard was specifically designed to make it easy to try out VR. The cost is low, and virtually any phone you have in your pocket today can power it. Content is even available on the Google Play Store - it can't get any simpler than that.
Pro Android Daydream ready
New Android "Daydream" ready devices (i.e. that meet certain criteria) get an improved VR experience with high performance sensors, high density, low-persistence displays (i.e. high refresh rate).
Pro Android Daydream ready
New Android "Daydream" ready devices (i.e. that meet certain criteria) get an improved VR experience with high performance sensors, high density, low-persistence displays (i.e. high refresh rate).
Pro Works with any Android phone
Depending on the version of cardboard, there may be some size limitations which limit your options. However, if you aren't picky about the hardware phones from 4" all the way to 7" are supported.
Cons
Con Requires an Android phone
The low price is somewhat deceptive, since it does require an Android phone to work. However, you might already have one.
Con Your experience will vary
Older hardware won't perform admirably, whether it be the SoC (system on a chip), display (resolution as well as sub-pixel arrangement make a huge impact on display quality for VR), or battery life (VR takes a lot of power).
Con Very basic
Google Cardboard was Google's way of generating interest for phone powered VR, and is literally just a piece of cardboard (or plastic) and glass lenses. The software is integrated into the Google Play Store, and Google more-or-less put Cardboard out there to see what developers would do with it. Later in 2016, Google will be making their next push forward with Daydream which will be a more modern, well thought through platform.
Con The official box is for Nexus 5 only
Even though you may apply the technique to other phones, the prebuilt one is only working well with Nexus 5 because of the screen size, NFC sensor, camera position, and magnetic sensor. However all of these are optional and it'll work as long as your phone screen size is roughly 5 inches.
[Additional Information:] Many phones can fit with minimal modifications to the Cardboard.