Superficial and episodic
To begin with, the chemistry between the lead pair is cringeworthy at best. And when that's the premise of your film, it's a major problem. The love affair that was probably the best kept secret of the princess' life, appears lop-sided, nearly one-sided for the most part. Maybe it was off limits for the filmmakers, but the lack of any deeper insight into the future queen's final struggles with the Buckingham palace adds to the film's unsatisfactory feel. However, it does throw light on the humanity and vulnerability of the people labeled as 'royalty', and that things can be very different from what they appear.
A+ for hair, make-up and costume design, although it doesn't quite guise the fact that Naomi Watts is almost a decade older than Diana was at that time. Nevertheless, she does an excellent job portraying the most famous woman of the 20th century. It's a shame the movie couldn't do her the same kind of justice and takes liberties embellishing (and tainting for the sake of drama) a very personal story.