There is a good collection of essays called Napoleon's Marshals that was edited by David G. Chandler, one of the best scholars on Napoleon. While it doesn't focus on their opinions, it does a great job analyzing the Marshalate. While out of print, it can be found for an acceptable price via Amazon.

I'm speaking of Napoleon I, if that was not clear.

As for scholarly, peer-reviewed articles, here are a few examples:

• Bertaud, Jean-Paul. "Napoleon's officers." Past and Present (1986): 91-111.

Bertraud's article delves into the officer corps as a whole, giving their social background and how the officer corp transitioned (or in some cases, not) from the Revolutionary army into that of the Empire.

• Dwyer, Philip G. "Public Remembering, Private Reminiscing: French Military Memoirs and the Revolutionary and Napoleonic Wars." French Historical Studies 33, no. 2 (2010): 231-258.

Dwyer, an accomplished scholar on the Napoleonic period, problematizes the Napoleonic memoirs. This is important when evaluating the opinions of Napoleon of the Grande Armée's veterans because many of them were recorded after Waterloo. A lot happened in French history that affected these memoirs. A memoir published during the July Monarchy would likely take a different approach to the great man than one published during the Second Empire, and Dwyer does a good job disentangling this facet of accounts of the Napoleonic period.

• Horward, Donald D. "Massena: Napoleon's Great Competitor." Consortium On Revolutionary Europe 1750-1850: Selected Papers (1997): 289-298.

André Masséna was considered one of the best of the Marshals along with Davout, but Massena also had one of the most strained relationships with the Emperor.

• Lamar, Glenn, and Gordon Bond. "King Jerome And Marshal Davout: Command Conflict In The Russian Campaign." Consortium On Revolutionary Europe 1750-1850: Proceedings 23, (January 1994): 120-128.

Although this article is not about Napoleon per se, it does examine how the Marshalate interacted with the Bonaparte family. In this case, Davout's significant conflict with Napoleon's playboy brother Jerome led to a critical mistake in the 1812 campaign.