I’ve done it twice- both times with not particularly serious cyclists (the wife, then my family).
It’s a very pleasant ride but there is nothing to test the legs. It’s all very gentle and the way to do it is to take in the views. The first time I did it there was a lot of flooding and we had a really trying time. We stayed in pensions mostly, they are very cheap and give you the option of showers and a decent breakfast (and, if you’re tight like we are, a lunch made of pinched bread rolls and ham). We’d probably not be together anymore if we’d camped, but then people were being rescued from 1st floor windows in boats.
The second time we did it fully supported, all hotels prebooked, all dinners prebooked and a van taking your kit to the next stop. It seemed to me a bit of a waste of money after doing it much more cheaply before. However, the knowledge of the tour company was useful for choosing where to ride and where not to. When we did it by ourselves we rode a really shitty bit of road into Linz and another into Krems, and the last 70km into Vienna, which weren’t nice. On the tour they booked you train tickets for these bits.
Rural Austria was lovely, as were Linz and Passau. People were kind, generous and friendly. As I stated I’d not camp, pensions are cheap and plentiful. Particular highlights for us were the middle of Krems (where we were in a church while it got hit by lightning) and the monastery at Melk.
I didn’t like Vienna either time. A very teutonic city with none of the charm of rural Austria, or the nice people.
The first time I did it we pressed on to Bratislava which was wonderful. A beautiful, friendly and welcoming city. Well worth one day less in Vienna and pushing on. I’d love to do the remaining stretch to at least Budapest one day.
There are some decent guide books about. Finding pensions beforehand in some of the more rural places can be a touch tricky but they are there. The nicest we stayed in for about £30 was Hotel Schwan in Traismauer.
We met a guy on it who had also done the North Sea cycle route which sounded good, and more testing if the bike riding is more important to you.
Let me know if you need more info.