This is one of the best roads I've driven. You turn off the dual carriageway in Monmouth and straight away get into some of the best scenery and roads you'll see in the UK.
The route passes through a few larger villages (ideal for stopping for lunch) and goes up and down the valley sides. You pass through Tintern and see the old abbey ruins on your right. This is a great place to stop for lunch and a walk.
The rest of the route from Tintern is fantastic, great tight corners and small straights mean that you can really enjoy your car without having to break the speed limit.
Watch out for cyclists though at holiday times, it's a popular route.