All other HomeKit motion sensors (like the Eve or the Hue sensors) are about 50 EUR, while the IKEA motion sensor is available for 10 EUR. Sure, you need a Tradfri Gateway, but that's 20 EUR right now. So even if you don't have a gateway yet, it's still cheaper than a single Eve motion sensor.
As you can see in the Tradfri Gateway Release Notes, the motion sensor and the shortcut button are now supported in Apple HomeKit. All you need is a Tradfri Gateway and the firmware version 1.13.21.
And that's it. After connecting the motion sensor to your Tradfri Gateway and the gateway to your HomeKit system, you have a fully working motion sensor that can turn your lights on and off or trigger any automation you like. The only "downside" I found is the appearance of the sensor. It's far from being beautiful. But that's a price I'm willing to pay. 😄