The Narita Express train is fast, clean & quiet. Some of the views are spectacular.
One word of warning, if you take this train and decide to get off at one of the bigger stations, say, Shinjuku; be careful to remember how to get back to your train stop.
Some of the stations are like small cities, with seemingly a small handful of stations combined into one. Maps can be hard to find and are deceptively oriented in the direction you're facing (instead of North like usual).
If I had a 12hr stop-over in NRT I'd definitely be doing a little trip out of the airport.