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disclosure: I've seen James McCartney perform live at a little club in San Francisco (Neck of the Woods). I saw Sean Ono Lennon perform once by accident - he was sitting in with The Flaming Lips.
I think the song by James and Sean is perfectly fine. It's a LOT more James than Sean. He doesn't have his father's voice, but who does?
BUT! That OUT-OF-TUNE G String on the guitar in the intro really takes me out of the song for a moment. Anyone can hear it, so I think it was on purpose