The one I got from Ebay is pretty well made. Many of my students have played around with it. My only issue was that the nose bridge was irritating, so I added a pad of foam, and duct tape.
There is quite a collection of free apps available for it, and more are being added. YouTube has several videos as well, under the #YT3D. Check out some of the YouTube 360 vids too, they're amazing on a phone or tablet. Google also has recently announced a partnership with Viewmaster, and if you're old like me, you'll know what that is.
I have had great luck with the apps, and my Samsung G5. At some point there will be apps for iPhones, but in reality, the iPhone is not keeping up as far as innovation, so I recommend getting an Android ASAP. If you wish to make old school Viewmaster reels, you can get a free app called 3DSteroid. There are probably others that are as good, or better, but I'm pretty happy with it. The videos (National Tunnel) below are examples that I made using this app. For teaching, it could make for fun projects