Descriptions on the website suggest the device will be pretty damn useful for avid players.
On catching Pokémon: "When a Pokémon is near, the light on the Pokémon GO Plus will flash. It will also flash and vibrate when you pass a PokéStop. Once a Pokémon is close, press the button on the Pokémon GO Plus to throw a Poké Ball. You'll only be able to throw a Poké Ball using Pokémon GO Plus if you've caught the Pokémon before. The Pokémon GO Plus will flash and vibrate to let you know if you were successful in catching the Pokémon."
On PokéStops: "The Pokémon GO Plus will begin to blink and vibrate whenever you're within range of a PokéStop. Press the Pokémon GO Plus button to search the PokéStop for items. If you find any items, swipe or tap them to add them to your inventory."