A Samsung account is required for use. You must create one during use or before. This is not compatible with Google cardboard as far as I am aware. This is only compatible with Samsung phones. Please refer to their website for a compatibility list.
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First, you can only access the Samsung and Oculus Gear VR app store with a Gear VR. The apps/games/experiences in this store are vastly superior to the Google Cardboard games and apps that are available in the Google Play store.
Besides the app store experience the hardware itself is different. For one, it locks your phone into the headset so it's a perfect fit every time. There are also hardware buttons that you don't usually have. The phone itself plugs into a USB port built into the headset. There are light sensors on the inside so it knows when you take it off and put it on - great to quickly pause an experience and come back to it. There is a touch pad on the right for tapping, swiping up and swiping down. There are volume controls on the right side, and there is a back button on the top. The phone in and out of the headset in order to navigate or really do anything at all. Some of them have a magnetic button, but many do not. This is much more than a single button too.
Holding the back button opens another menu to adjust volume as well as brightness. You can also "reset" or recalibrate the screen. So if you change positions, you can reposition the screen you're looking at. This is really handy when you want to face a particular direction. It's essentially a global tools menu that allows you to adjust some settings while in-apps.
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The Gear VR only supports ocular adjustments front-back, and not side to side for eye width. So far, this hasn't been an issue at-all.
After a prolonged time using my Galaxy S7 Edge in the headset it can overheat, to where I get a warning message. Taking it out for a half an hour solves this - however I can go hours without having an issue. It's definitely not the "15 minutes" I've seen some people experience with the last-gen hardware.
There are some really good games on the Gear VR. Some of them require a gamepad, but so far it's been worth it! I've flown through space in End Space, fished with Bait! (which is actually pretty fun), been scared in Dreadhalls, reeled in Rilix Coaster, and hack-and-slashed in Herobound: Spirit Champion. All of these games are a ton of fun and highly recommended. The coaster and Bait! are both free, so no cost at all. There are several other free and pay games that are worthwhile, but those are my go-to notable ones to demo the app with. Samsung VR is a video-viewing app that has stock videos updated frequently (streamed) along with the ability to play downloaded videos.
Samsung Gear VR - Virtual Reality Headset specifications
Dimension 201.9 x 116.4 x 92.6 mm (318g)
Field of View 96˚
PUI / Button Touchpad, Back key, Volume Key, Focus adjustment wheel
Sensors Gyro, Accelerometer, Proximity
Charging port Micro USB (for mobile phone)
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Samsung Gear VR compatible devices S7, S7 edge, Note5, S6 edge+, S6, S6 edge (*Note5 and S6 edge+ may require software update).