I've ridden two sections.
1) Donauquelle in Donaueschingen to Ulm. Surface was mixed, mostly decent tarmac but some gravel paths (but manageable on a laden tourer). Also some pretty vicious short, sharp climbs. Extremely pretty.
2) Near Linz, Austria. Surface was good, very wide path. Problem is there are lots of OAPs on eBikes and families weaving around. It might get a bit boring after a few days?