Thanks all for the input, much appreciated. I like the sound of Tuscany, Chris. I understand it's a beautiful area.

Ollyisk, I've cycled the Rhein and the Mosel, which is close to Frankfurt. From memory, when researching the route, it can be a bit industrial around Frankfurt, after that it turns nicer, but is never really nice until you hit the mosel. The problem with the Rhein, I found, that far up, is that everything is crammed into the valley - that is, road, river, radweg and railway. The towns can be pretty, but also functional. Koblenz, like many places in Western Germany, was flattened in the war and rebuilt in a generic western style. But once you turn onto the mosel, things get better. It's very placid, very pretty - genteel is the best word. Trier, at the end, is a great town, and there are various routes (the Kyll for example), which take you off the mosel and as such allow you to do more of a circular route on the way back. The Hunsruck, I found, was peculiar in that, around Hahn, it was pretty ugly, but once away from there, again, prettiness and gentility reigned.
The Rhein farther south of Frankfurk is really beautiful. There are some great towns just off it (Karslruhr being one - where the bicycle was invented). Farther south, you get into the Black Forest, which is stunning (visit Freiburg - one of my favourite German towns). The Rhein bends east at this point and is spectacular, though only on the German side, I found. A quick trip to Switzerland left me somewhat less than impressed.
I haven't cycled east of Frankfurt, so can't really comment, but one thing I would say is that, for me at least, Germany beats most countries hands down when it comes to cycling. The radweg system is amazing, seemingly linking even the smallest hamlets, the people are friendly, the hotels accommodating, the scenery wonderful. For me it's a great, great country, which is luckily overlooked by most tourists. Long my it stay that way.