Is it worth a trip just to see the Guggenheim?
If you like architecture, yes.
The actual artwork is on the mediocre side.
Bilbao itself is an industrial/port city. Suspension ferry at mouth of the Nervion was designed by Eiffel (or one of his deputies, forget which).
Cave paintings further west in Altimira/Santillana are as good as those in southern France.
Depends where you're coming from. If it's from afar afield as Barcelona or Madrid, then make it part of a couple of days that would include San Sebastian and, as Ed noted, a stop at the Altamira caves. But if you have no inclination to visit the Basque country otherwise, then no, it's not.