To add to this - this means it could be enabled in focus/klar nightlies, except they don't have an about:config page. You can build it from source, though!

It's very early days currently so you're not missing anything, but as it gets more stable I'm sure it'll be easier to try out.

I keep wondering about GeckoView vs Fennec. Surely Fennec already contains something much like GeckoView, so what will the difference be and when will (Nightly) Fennec switch?

GeckoView and Fennec share the same Gecko code, but GeckoView provides an embedding API for other apps and has some runtime differences like enabling e10s (sandboxed content process separate from the app UI). GeckoView-based features (like e10s and WebRender) will eventually be folded back into Fennec.