Tulard has always been the yardstick for memoirs, and folks should pay attention to him, I think. As I recall, he aid Talleyrand's "memoirs" were a fraud, beginning to end. But I also think we can use these memoirs--except for Talleyrand and Fouche--for the interesting facts about who was where when, and how Madame X looked at the Tuileries, or that General Z had a drinking problem. Always wise, though, to compare the gossip with other gossip.
As for using anything Junot, Savary, Marmont, de Remusat, and Marbot said about policy, diplomacy, warfare in general, or any issue more substantive than what we might find in the Daily Mail or National Inquirer, probably a risky move.
I did like what Laure Junot said about the Queen of Portugal being the single ugliest woman in Europe though.