I think Woody Guthrie was a good singer rather than a great one - and of course, he was no great shakes as a musician. However, I think relatively few people witnessed was he was really good at, which was the ability to really thrill people in practically any close up, spontaneous live situation. He was what I would call a busker through and through - and he must have been bloody brilliant at it by all accounts. I like his writing too.
Oddly enough, when he met Bob Dylan, he rated the youngster more as a singer than as a writer.