Unfortunately, while Flynn certainly has a great deal of charisma, he doesn't quite have Cary Grant's air of sophistication, and the script isn't quite tight enough to make up for that. But the movie is fun and enjoyable anyway, and at some points it almost turns into "Spot the Cameo!" as a variety of stars come wandering in and out of the picture, including (but not limited to) Walter Connolly as a cantankerous old millionaire (basically the same role he played in It Happened One Night), Melville Cooper (the Sheriff of Nottingham) as a butler, Margaret "Wicked Witch of the West" Hamilton as a housekeeper, and Hugh Herbert (a popular vaudeville and radio star with a tendency to bridge his fingers and make a distinctive "Hoo-hoo!" sounding laugh, often parodied in cartoons). Supposedly Lana Turner has a bit part as well, but I didn't notice her ... and of course, it was directed by Michael Curtiz. The only person missing is Alan Hale!
Errol Flynn always wanted to do more comedies, but of course Hollywood pretty much only saw him swinging around in tights. Most of his roles do let him have a few comedic moments, and Gentleman Jim is basically a comedy with biopic trappings, but it was fun to see him stretch this way. As laurie_robey put it, "A role like this must have been a breath of fresh air to him!"