I know absolutely nothing about Glenn Miller and from watching this film I didn't pick up much, but I did really enjoy Jimmy Stewart's performance and since he was the main reason I watched the movie I walked away satisfied.
The film opens with a young man, Glenn, down on his luck. He's traveling around, playing in various bands, but they just don't have "the sound" that he likes. Things continue on this way for about an hour until finally he gets his big break with a song he composed called "Paraplegic Pussy Pounder" whoops sorry that was Sexcrement, not Glenn Miller. Sorry. I meant to say to say "Moonlight Serenade". And, maybe I'm showing my ignorance here, but it sounded a whole lot like the stuff he was poo-pooing earlier. I guess it had a bigger sound and was more polished, but that was the only noticeable difference I heard.
I would complain about the story being too quick and lightweight, but I think the intention of the filmmakers was to make a movie about the music not a movie about the man. And in that respect it did a good job. So good in fact that I've been listening to the Sirius "40's on 4" nonstop for like two days! I'm actually really digging this stuff. Which shouldn't come as too much of a surprise since I've always enjoyed the music Woody Allen uses in his films.
Back to the film...it's lightweight, but enjoyable and Jimmy does an excellent job as always. I can't say the same about June Allyson, who I've never been a fan of. Whether you watch it or not really depends on your interest in the subject matter or, like me, your interest in James Stewart. That said (now that I've done some reading about Glenn online) I would actually enjoy seeing a serious film about his life and mysterious death.