In the end, it depends a lot on your preferences and use case scenarios.
I have an Apple Watch and use it for running. Out of the box, it is vastly inferior to most Garmin watches. However, there are really good apps that lift it to a similar level as a real GPS sportswatch. I use WorkOutDoors and HealthFit and I think it adds a lot of features you would otherwise miss:
WorkOutDoors is my workout planning, tracking and routemapping tool, it also has great analysis features in the iPhone app.
With this app, you can plan structured workouts, it has audio-guidance through each intervals, you set a target range for pace, heart rate, cadence etc... It has configurable screens with a lot of metrics, so you can choose to display all kinds of data such as cadence, interval paces, configurable rolling paces, step length and so on and so on just on your tracking screen on the watch.
Also, you can show a map on your watch screen and import GPX-files to show your path while walking (I only use that while hiking, not for running).
With the HealthFit app (RunGap does similar stuff), you can then (automatically) syncronise your activities to all kind of platforms (Strava, Runalyze, TrainingPeaks, I believe GarminConnect as well etc...)
So, of course if you need all the physical buttons, the watch is lacking that. Also, the battery life is much shorter than on most Garmin watches. And it might take some time to set up those apps, it does not work as a running watch straight out of the box. If you go mountain climbing, a garmin is also more robust than an apple watch.
But, there are those people who religiously defend Garmin watches and say an Apple watch is never really a sports watch, but they can not really give a conclusive list of what features the Apple watch with the right app is missing...