Navigation is as good as the mapping really. It works great on my Edge Touring (with free openstreetmap) for road stuff, much like a car satnav it shows the junction coming up, along with road names. Have only used it once to direct me without a pre-planned route, it did fine but had a preference for quieter roads and cycle paths even if it was a bit longer.
I used a Edge 200 for quite a while (and a forerunner before that), following a line on a map gets you there OK but plenty of junctions aren’t obviously shaped and I’d overshoot a few times a ride if it was somewhere unfamiliar. It beeps at you after a few seconds, and it’s easy to get back on track, but enough to justify the cost of a Touring to me.
Off-road the line is usually good enough to figure it out, but if you’ve got mapping that understands turns then that’s handy. Nice too to work out a quicker way back if you’re cold/knackered/it’s getting dark.