I'm an early adopter of HomeKit, and I must sadly admit that HomeKit is a half-baked disaster. Not everything that Apple created under Tim Cook is a disaster, though. Apple Pay is brilliant in my opinion, and those who have compatible hardware but never bothered to use Apple Pay should really try it because it's a revolutionary solution. However, HomeKit is a prime example of how wrong things are getting within Apple.
The solution was never explained to the user base. It's unclear where HomeKit actually lives (cloud or iOS devices), it came out as a framework without an app, the recently released Apple Home app is inferior to other HomeKit apps, some products that are perfectly functional with a third-party Home app do not function well with the Apple Home app, the native HomeKit geofencing API has not been open to third parties, so they have to implement their own geofencing solutions, the native HomeKit scheduling API does not allow for the creation of weekend schedules that differ from weekday schedules, and the list goes on. Siri cannot trigger scenes half of the times, and the while experience is extremely frustrating. The whole idea behind HomeKit was that smart appliances can be controlled in one place - initially through Siri only. Unfortunately, this has never worked properly, and so I find that controlling my smart appliances via their respective apps is a much more convenient and reliable way to interact with them than trying to use HomeKit (via Siri, Apple Home app, or even third-party Home app). And when HomeKit is broken, and your invitees can no longer control the devices, the only solution that Apple provides is to reset your HomeKit and start from scratch, which can mean multiple hours of reconfiguration just to get the basic functionality working again.
The case in point is the absolutely ingenious ecobee thermostat product that is absolutely crippled by HomeKit and is brilliantly implemented via the native ecobee app. More complex features, such as setting vacation schedules are not even possible in HomeKit, but this is exactly the kind of feature that makes smart thermostats stand out from dumb thermostats.
The extra surcharge for the ecobee thermostat compatible with HomeKit is an absolute waste of money as the ecobee thermostat without the HomeKit chip functions brilliantly through its own app, and the optional HomeKit chip adds no value whatsoever to the user experience as most users will eventually go back to using the native ecobee app.
Same goes for Hue lights, iDevices, etc.