While no reasonable reviewer would give any B-detective series movie a score of 10 (after all, they were not exactly deep and they tended to focus much more on entertainment instead of originality), for such a film, this Lone Wolf film scores a bullseye. As far as quality and watchability goes, I would place it on par with the average Sherlock Holmes, Charlie Chan or Saint film of the era. In fact, in many ways, this film seems a lot like one of the Saint films because the hero is a reformed gentleman thief who investigates crimes for kicks--and chases women along the way. While the film starred Warren William, I could have just as easily imagined Tom Conway or George Sanders (from the Saint and Falcon detective series) playing the lead in this film. This isn't meant as a negative per se--it's a very familiar but also very satisfying formula.
Warren William was wonderful in the lead but he was also ably supported by a better than usual supporting cast. Having Ida Lupino AND Rita Hayworth playing the female leads was pretty amazing, as neither were exactly stars at the time and it was just a case of dumb luck in casting. In addition, the other supporting players all provided some needed depth to the film--something often lacking in B-movies.
As far as the plot goes, it was pretty interesting but also purely secondary--not at all unusual for this type of film. You certainly don't watch this type of film because of the finely crafted mystery, but more for the personalities and style. In many ways, this sort of film is like a warm and familiar pair of old shoes you put on after a long day at work.