I gave Till the Clouds Roll By a 5/10. It's worth seeing for the music, and I'd have rated it higher had it been shortened to just the music. It could have got another bump by allocating the entire time to performances -- there was time for over twice as many songs, and there's no problem finding enough good Kern songs to fill the time. Kern wrote well over 1000 songs, and though of course not all are memorable, finding 50 or 100 good ones is easy.
But the choice and arrangements are nearly random. The order in which the songs are presented makes little musical or dramatic sense. They are done by a variety of performers with no consistency. The time after Show Boat is mostly ignored -- hey folks, Kern was actively writing for another 15 years and did his best work from Show Boat on. So even had the movie contained a full three hours of music, the best I could call it would be a haphazard revue with very good performers and memorable songs.
The worst, as others have pointed out, is the story line. To make it clear: THIS STORY LINE IS ALMOST ENTIRELY FICTITIOUS. Oh, of course the shows mentioned are accurately described. But I own a biography of Kern, and the name "Hessler" does not appear -- the character of Jim Hessler is totally fictitious. Kern's wife Eva was British, but the circumstances of their meeting do not resemble the movie. It's probably true that Kern just missed sailing on the Lusitania, but for totally different reasons (Frohman planned the trip for both of them, and Kern overslept). Etc etc ad nauseum. Why bother? The movie would have been much improved by letting the music speak for itself.
Edward