We took the Avalon trip down the Danube in June 2012, our first river cruise, and it would be hard to imagine how it could have been better. The staff were excellent; the cruise director was a mine of information, and the tour guides in each city were excellent too.
We stopped in several towns and usually had a good few hours to look around (Regensburg was a bit hurried--a quick look at the Cathedral, a quick look at the outside of the Albrecht Durer house, a stop in a drugstore for some cheap razors, and browsing the market stalls with fragrant berries and gluten-free gingerbread). We skipped most of the optional tours so that we could relax on our vacation, and nothing beats being lulled to sleep by the sound of water lapping at the ship, as we cruise down the Danube with our picture window open. (Of course, when we docked or left our cabin, we'd keep our window locked.)
The food was excellent, and the wine at dinner flowed freely. The entertainment was generally enjoyable, but the highlight was a couple of musicians from the Bratislava Opera (I think) who played a range of classical and folk instruments. The Tranquility was astonishingly quiet--no noise of engines or other passengers disturbed us in our cabin. One nit to pick: some of the life vests had an electric light dated 2011 instead of 2012.