The novella is hilarious. Lady Susan is the wooorst, but you can't help but cheer for her as she makes sure that everyone does exactly what she would like them to do. I would love to see her go head-to-head with Cersei Lanister or Margary Tyrell. They would eat each other alive, and it would be a delight to watch. She's a master at reputation control, and her ability to explain things so that people think the best of her and the worst of her enemies is amazing to view. Lady Susan is unique for Austin heroines, because she is the woooorst, in the other novels she would most likely have been the villain and her daughter Francesca the heroine. And though Francesca does get the heroines ending, Lady Susan isn't really punished for her behavior either. It's really quite a feminist novel in that it explores the ways that a woman can have power in a heavily patriarchal society. The fact that Lady Susan isn't really punished for her uses of soft power is what makes it stand out against other novels from the late 1700s early 1800s that dealt with manipulative women. (Vanity Fair, I'm looking directly at you.)
While I quite enjoyed the book, the end is very rushed and Austin completely gives up on the format, likely because she grew frustrated with the limits imposed on her by the chosen format. There is after all, only so much one can tell with a letter and have the letter still be a believable letter. It's a very short book though.
As for the movie Love and Friendship, the movie based on this book? Having read the book afterwards, I can say it's a pretty true adaption. They had to pad out a lot of scenes, and stated some facts where the book only heavily implied them. However, it's a decent adaptation.