Hi all, first time posting, (though I've lurked here for a long time) so   please be gentle with me!
I give it about a 7. I had heard a lot about the movie and maybe I had anticipated that it would be funnier; I enjoyed the earlier Guest movies: Spinal Tap, Guffman, Best in Show.
I do remember the era and the groups they are parodying, although I was young at the time, I remember the Limeliters, the Highwaymen, the Kingston Trio, and as for the 'neuftet' the New Main Street Singers, obviously they reminded me of the New Christy Minstrels, and weren't the Serendipity Singers similar too?
Guest's tenor voice reminded me of who? Glenn Yarborough? It was a very familiar vocal style that Guest did effectively.
The humor was pretty subtle, but I thought some of the songs were surprisingly catchy, (like 'Never Did No Wanderin') and very true to the folk-pop genre.