I've never seen the film....
but was the singing truly that bad? If I ever got to the chance to watch this, it would be just to judge the singing.
They survived the impossible...by doing the unthinkable.
No, and yet, yes. The Broadway cast album is wonderfully sung by Julie Andrews and Robert Goulet, and Richard Burton does very well in spoken voice. As for the movie .... the world is divided on Richard Harris - I like his singing, myself - but not everyone does. Guenevere's songs don't have as much impact, they suffer from very poor phrasing, and are probably the most disappointing. Whoever dubbed Franco Nero was okay - maybe better than okay - but miming is still miming, and on Broadway Goulet had a stunning voice. It also feels as if an effort was made to infuse a sort of 'hipness' into the movie, and that impacted the singing style and to some extent the score. A number of songs were cut, or reinterpreted. Joshua Logan seems to know as much about musicals as a fish knows about rollerblades.
That said, the music is still wonderful, the production design is lovely, and the actors are generally charismatic. Harris' singing is heartfelt and at least interesting, but no guarantee that you'll like it. As a whole, it suffers by comparison with the Broadway cast, but it has a certain breezy charm. As you can perhaps tell I'm fond of the film, but conflicted. Do watch the first half hour or so if you get a chance. The second act is a real downer, but the opening is often lovely and even engaging.