I had never heard of this movie until just catching it on TCM. What a pleasant surprise, as I've always loved alternative reality stories, whether "It's A Wonderful Life, the "Back to the Future" series or the several Twilight Zone episodes that this movie seemed to be a model for.
This is not literally a time travel movie, and I kind of like that it's made clear this is all a dream (induced by anesthesia). But it still has a lot of dramatic impact. The idea of "what if I had my life to live over again, knowing what I know now" seems to have a universal appeal. The very clever screenplay spins several variations on this theme, and even if the "lesson" learned by the hero is predictable, there are enough plots twists to maintain viewer interest. And as another reviewer commented, the script makes great use of true historical event. This movie is actually a pretty good history lesson. Probably a lot of viewers are unaware that our entry into World War I was very controversial and not at all universally favored at the time.
The script is particularly clever in it's parallel construction between the "real" story and the dream. The roles of the rich guy vs. the struggling storekeeper are reversed, but in both cases, there is the concept of changing one's life with a bold and maybe risky investment of one's life savings.
My only quibble is that the movie seems to end abruptly after the Lee Tracy character regains consciousness and finds that he's still married to Mary and back to reality. I actually thought there would be another plot twist, with him discovering that Otto Kruger is a con man trying to get his $4,000, and not really a rich success. That would've added another wrinkle to the "be happy with what you have" message.