It's about 50-50 between quiet road and dirt path. The path is not rocky and reasonably easy to cycle on.
It's great to get back on the 'real' camino after the crowds on the Sarria - SdeC leg on the
camino Frances.
There are some lovely, lonely high moors between Olveiroa and Cee. The woods west of Negreria are nice too.
My bike is a Koga Randonneur treking bike
http://www.koga.com/en/bikes/trekking/collection/randonneur.htm?frame=H.
If you ever want to take over the world this is the bike for the job! I had cycled from Clonmacnoise in Ireland, down the west coast of France and
Camino Frances and got two punctures. No other problems whatsoever.
This bike is totally overkill for the Santiago - Finnisterre leg. You can do it on any good quality bike. If I was only doing SdeC, Finnisterre, Muxia, SdeC again, (don't tempt me!) I'd consider taking my road bike.