I've been on the Main twice and thoroughly enjoyed it. Five years ago with Teeming (Nuremburg, Bamberg, Wurzberg, Wertheim, Miltenberg, Dusseldorf and I can't remember the others!), and last year with Viking on the Main and Mosel, starting in Prague, bussed to Nuremberg, then on to Bamberg, Wurzberg, Heidelberg, Mainz, Cochem, Bernkastel and Trier. That being said , I think of the rivers I've been on (also the Rhine, Danube and the Rhone), the Douro had the most spectacular scenery.