My girlfriend and I cycled from Passau to Budapest in 2006. We took only this book, the Bikeline map, a tent, and hired bikes in Vienna. The route is very flat and well maintained (on the Austrian side), suitable for all levels of cyclist. We had a great time and would recommend it to anyone.
Mr Higginson has done a fine job of covering the journey. Every village you pass through on the way has at least a couple of paragraphs dedicated to it, briefly describing the history of the town, sites to see, places to stay and eat, and maps for the larger towns. There are many interesting things to see along the route, so pace yourself - you could easily spend a whole week in Vienna alone!
There are a few minor inaccuracies in the book due to recent changes. Currency isn't a big problem - Euros and credit cards are usable in Austria and Bratislava, and there are plenty of ATMs. After crossing the border into Hungary the book warns that you will have no opportunity to get Hungarian Forints; the first town you pass through now has an ATM. The exit from Gyor is described as being unsignposted - now someone has helpfully slapped navigation arrows along a series of lampposts. The route into Budapest is described as being a "nightmare" through dangerous roads filled with busy traffic - now there is now a pleasant cycle route from Obuda into the heart of Budapest, which conveniently passes the Obuda campsite.
I only have minor criticisms of the book itself. The photos are too dark (cloudy/poor lighting). The focus is often on the local church and other historical attractions; it misses some of the more exciting attractions like outdoor water parks that may be interesting to younger cyclists. Maybe something for the 2nd edition!