Melk - itself is a cute little town under the shadow of the giant abbey. Its a friendly and had a nice Christmas market with singing. There were a couple of nice coffee shops. We came back twice. The abbey looks like a big library above the town - not as ancient as I expected. You can get to the abbey by climbing some stairs from down town, or else drive around to a dedicated parking lot. We came back twice. We found out later than there was a Nazi camp in Melk which dampened the memory.
Krems - A huge disappointment. We kept wondering where is the good part of this town. The photos of the hotel seemed to suggest a patio view of the river, but actually it was a patio view of a drainage ditch or off shoot, and the town is a bit far form the river. A lot of its pretty industrialized. The city center had the worst Xmas market - not a single thing being sold except liquor and everyone getting drunk. No food, crafts, or other things.
Durnstein - this is an interesting place. But the "castle" is a pile of rubble on the hill. There is a nice chapel there - which seems like a miniature Melk abbey. If you drive its a twisted narrow set of roads, and the way to get out was to pass all the way through.
The overall river view between Melk and Krems - not really very exciting at all. Fairly flat land with a wide river. I would say that the Rhine in Germany was much more scenic. Its not a bad area, and maybe its the "most beautiful stretch of the Danube", but its not at all worth any extra effort to be there or see it.
If we were to do it over again, we would drive to Melk and spend a few hours, then move on.