It depends on your definition of "good." Do you mean from a "taste" or "health" perspective?
Mussels are filter feeders and they will taste similar to the substrate and quality of water in which they reside. They also will bioaccumulate heavy metals and other contaminants to a certain degree.
With that said, if you harvest mussels from a relatively clean water system and don't eat too many at a time; they can safely be consumed when taken from Idaho waters.