How bad is it? It's a kitschy disco-era musical.
Should you see it? I don't think so.
A local band (BeeGees plus Peter Frampton) finds success, but must battle corporate types and an evil rival band (Aerosmith). George Burns narrates, but the rest of the dialogue is sung, mostly to Beatles songs, though the Beatles are barely mentioned - one character is named Strawberry Fields, for example. Donald Pleasance disco dances, but otherwise, there's not much that's memorable.