We only managed to visit the Southern part of the valley, from Bingen to Bacharach, however we really enjoyed the experience, and felt we got a feel for the site as a whole.
We spent a night in Bacharach, in the fantastic Castle Stahleck, which sits on the steep hills above the village. The castle is a tourist site in it's own right featuring on most of the tourist literature for the valley, and well worth the sometimes treacherous hike up from the river. It is now a Youth Hostel, and ranks up there with the most impressive and unique budget accommodation I have stayed in (A cave under the Sahara-cum-Star Wars set in Tunisia, may just beat it though). It has a massively impressive wine list, testament to the incredibly steep vineyards that line the whole of the valley. These vineyards are an integral part of the landscape and culture, and a few glasses of the local Rieslings really complete the experience of visiting the valley.
The village of Bacharach is incredibly pretty with lots of half-timbered buildings (picture with vineyard in background), and cobbled streets. It was very quiet on a Thursday morning in January, but I gather it gets very busy in summer. We missed out on having a boat trip as we couldn't quite fit it into our itinerary, but this would really be a lovely way to experience the area.
The villages on the western bank of the river are linked by a great train line, which runs along the bank providing great views. There seems to be two trains an hour from most of the stations between Koblenz and Mainz.
I really enjoyed the southern half of the Valley and it is a very easy place to visit, within easy reach of Cologne in the north and Frankfurt in the south. It could take up as much time as you would like to give it, and Castle Stahleck is a real highlight and I can really recommend staying a night there.