How bad is it? Depends on how much you like the music. Somewhat below average, I'd say.
Should you see it? I've already recommended Skatetown USA and Roller Boogie, so... yes.
Olivia Newton-John plays a Greek muse (Catastrophe, perhaps?) that comes to life and convinces a man to open a roller disco. Gene Kelly dances in a giant pinball machine. Sandahl Bergman has a role, as does Wilfrid Hyde-White's voice. There's even an animated sequence. The movie is essentially a 90 minute music video featuring songs from ON-J's post- "Grease" album.