We can't gush enough about the Antoinette and the service we received there. The staff was so helpful and friendly, including accommodating my husband's serious food allergy (the chef even baked my husband a cake for his birthday! first time he's ever been brought a cake and had waiters sing to him in a restaurant -- usually if he's going to have a cake, I have to bake it!). We were very comfortable in our cabin, and also really enjoyed playing board games in the pretty lounge, basking on the sun deck (the "Romantic Rhine" stretch of cruising with all the castles was really incredible), and swimming in the tiny pool (be warned that it is usually quite cold). We thought we'd be really excited about the movie theater, but we didn't want to see any of the movies being shown there.
The food was great, especially the "epicurean dinner" night. We wished there were tables for 2, but had nice conversations with the other passengers who sat with us (at breakfast and lunch we were usually able to have a table to ourselves).
The itinerary itself was not the big draw for us, the boat was. We did a lot of wandering on our own, and found Koblenz and Strasbourg to be the most interesting towns to see by walking around. Rudesheim was the least interesting of the places we stopped.
We weren't bothered by the engine noise even though we were the last cabin at the back -- it was a low hum and not hard to sleep through. We would really recommend category 3, since it was wonderful to have a window that opened. The cabin did not seem small, the closet and drawers were adequate and we were particularly pleasantly surprised that there was a roomy drawer for toiletries in the bathroom and plenty of counter space.
Lots to see, very scenic.