I love Georgette Heyer's books and was disappointed in The Black Moth at first, as it lacks her usual sparkle. However, before I finished it I found out it is the first book she wrote, when she was only 15, which is a different matter altogether. There are too many main characters, and the plot takes a long time to really get going, but there are glimpses of the fine author she would become.
If you like Georgette Heyer and would find it interesting to examine her first book, I recommend it. Otherwise, I recommend you pick up one of her other regency romances.