I can understand why Diana might be a disappointment for most.
If one is after a detailed portrait of "The Loneliness of the Disaffected Princess" then this film will only partially fulfill.
If, however, one is after a simple, easy to watch love story about the affair so well hidden from the public eye in the 18 months prior to her sudden death, then fulfilled you shall be.
The disappointment for me came from the fact Diana's director, Oliver Hirschbiegel, created Downfall, the depiction of Hitler's final days. The film was an Oscar nominee, a deserved reward for its depth. Diana is his first foray into a love story. He talks of his research coming from the 2004 inquiry and The Kate Snell book of the affair. I suggest that the details coming from those sources was light and frothy.
The film is pleasant enough with a shallowness to match a children's wading pool, but as I say, it depends on what you are expecting. Oh, and Naomi Watts is lovely but not quite as beautiful as The Princess we remember!