I'm dumbfounded at the criticism of this episode. I think it gets better and better with each viewing. There is enough about the plot on other posts, so I won't go into it other than to say it is compelling and not absent.
Joan Fontaine's performance is exceptional and she embodies in a short span of time the type of person that would truly drive the most gentle of souls to murder. The character is so tone deaf to anything but her own whims that it makes your skin crawl and has a cringe factor that I've only experienced with a handful of Hitchcocks or Suspense episodes.
It's brilliant and certainly a top performance -- not only by Fontaine, but her husband in the show also. I highly recommend it.