There is a similar discussion on the main page- "Can f18" thread.

I've only started using gps. Basically, I'm just using it as a knot meter. If my speed slows, then I'm probably getting headed or pinching. If it increases I'm probably footed off too far. It takes a fraction of a second to look at the instrument and a second or two to determine what action to take.

Mostly my head is out of the boat, when I glance at the horizon and that has changed, I look at the instrument.