Depends on what model, what your workouts are, how active you are during daily life, and how default you leave it at.

Any situation can be improved by tweaking settings and using better - the amount of manual intervention will depend on the above things.
Could be a matter of set it and forget it - just use the numbers.
Could be a matter of logging every one of your workouts manually.

And you could get comments of terrible accuracy from someone using it totally wrong and of course it won't be right, you could get comments of great accuracy and they also don't realize why it works so well for them.

I'll say with my understanding of it, a simple non-HR device, I did a month of triathlon training with lots of workouts and Garmin for better calorie workout burn, and based on weight actually lost from accurate food logging - was less than 5% off.