Amazon has always been a bit odd about providing its Prime Instant Video service on Android devices. Support for it has always been baked into Amazon's Fire devices, and there was an app for iOS too, but for Android at large, nothing. Nada. Zip.
A while ago Amazon added streaming support to its main shopping app. This, again, was unlike the process in iOS and on Fire devices, but at least there was a way to watch things you'd bought or stuff you were able to stream from your Prime Instant Video account.
Now though, Amazon has launched an app for tablets specifically, but that also works on phones. It's a stand-alone app, but again, it's weird. For one thing, when you open it you are simply redirected to the Amazon website. From there you're able to search for a program or film on the service, but it's not optimised for your device at all. This makes pretty much no sense.
Long time coming
When you select a video to play, the app will then spawn its own player, but as with all Android apps, you'll be asked if you want to open it in your browser (sometimes two browsers if you have Chrome installed) or in the app. You can save your preference, but none of this is exactly user-friendly.
If you install the app on a phone, Amazon suggests that its existing app is more user-friendly, and gives more features, like playlists.
The biggest problem of all though? Still no Chromecast support. You can cast your whole screen, but that's really not the same. Come on Amazon, we know you make competing products, but can you please just let us enjoy the things we've paid for on our choice of devices?