Force touch is useful sometimes, but the problem is you never know where it's available until you try it. Yes you eventually learn where it's available, but that can be a challenge for the apps I don't use that often. If you don't think to try it, you might not even realize every function available in a given app.
My most frequent use of Force Touch has been to change watch faces, but I will be happy to do away with it and just swipe side to side instead after upgrading to OS3. I guess I also use it to pause workouts, check weekly activity stats, and my other most frequent use for it: clearing all notifications.
I think the one consistent use for it I have seen in most apps is that it's usually the way you add a new entry for something in the app.
Sean