I knew next to nothing about the Kennedys before watching this series. All I knew was that John Kennedy was assassinated and that he had an infamous affair with Marilyn Monroe. If you're the same way, I would watch The Kennedys for a mini-history-lesson.
The one thing that attracted me the most was the acting. I've seen some of Greg Kinnear's work before and enjoyed his performances, but I was quite unfamiliar with Katie Holmes, and I personally think she was great as Jackie Kennedy, and also Diana Hardcastle as Rose Kennedy. Even Charlotte Sullivan as Marilyn Monroe had several bright scenes in her performance.
I mentioned above that I knew JFK was one romantically involved with Monroe, but I was frankly surprised that it was made to look as if her death was a suicide after Bobby Kennedy ended their affair. I also didn't know the Kennedys had any contact with Frank Sinatra- but I wasn't impressed by Chris Diamantopoulos, the actor chosen to portray him.
Overall, The Kennedys is a solid portrayal of the notorious political family. I recommend it to anyone who's considered viewing it.