I had not experienced 3D Touch properly until I got my iPhone X to see how it fits into my usage.
The feature I probably use most is 3D Touch on Settings icon to go straight to battery to see screen on time usage.
I am more than willing to use 3D Touch more on other apps from the home screen at the very least, but at least on stock iOS apps they don’t provide a great experience.
The second one that deters me from using it, is the messages app.
After I got my phone, 3D Touching on the Messages app would bring up people I messaged recently, but they might not be the people I’d consider messaging the most and the ordering can be different etc.
I hope Apple adds support for adding a fixed lists of up to 3 people you message frequently and in the order you want, to know with confidence that when you 3D Touch the messages all that you will get to the contact you want.