There has been a number of biopics made about Marilyn over the years but The Secret Life of Marilyn Monroe covers the parts of Marilyn's life that she did not want made public. However with such an icon Marilyn's story is well known and this drama does not really cover any new ground that we haven't seen in other biopics of the star.
Being only a 2 part made for TV bio-pic only certain parts of Marilyn's story could be covered and the biopic does not really go into Marilyn's marriages in any depth nor does it cover some of her biggest films like some like it hot. It also fails to mention Marilyn's relationship with Frank Sinatra.
There is also a number of inaccuracies that occur in the biopic (you can read more about those here)
I also found the final scene with Marilyn on the beach with her mother a little contrived and 'fluffy'. Having said that this show is well worth a watch as long as you don't really expect too much of it.