Engie comes in 2 parts, an app that can be downloaded from an App Store (both iOS and Android versions are available) and the device that plugs into the OBD port (the Android and iOS devices differ). It's possible to download the app for free and set-up an account and order a device through the app (which sends you to the website) or just order the device from the site directly.
Once the device is plugged into the OBD port and the engine turned on, launch the app, search for the device and then connect. The app will then show how the car is performing. There's various modes which can show things like engine temperature, actual trip costs (using real petrol pricing that you have entered), however the real USP of the app is that if there is a fault, Engie will tell you what it is and can then send you to a local garage - the app knows where you are and has a large garage database.
The only downside is that if your car doesn't have any faults, then there's no real advantage to using Engie compared to other OBD devices and other free software that's out there.
The Android device is £14.99 and the iOS device is £19.99 available directly from Engie (it arrives quickly once ordered).