This is not a bike path, but a (long-distance) bicycle route. In my experience, those consist of a mix of quiet roads (usually tarmac that has seen better days), some gravel bits, old railway lines converted to bike path (best case scenario) and occasionally busier roads with a decent bike path running alongside the road. No personal experience, but the bicycle routes I've done in Germany were enjoyable (especially the North Sea one from the Dutch border to the Danish border, although the bit through the Netherlands is far superior).