They usually have access to materials that aren't available to anyone else, so often they produce excellent work (that's then not declassified for 30 years, in some cases). Mostly these folks serve different purposes than writing objective history for academic consumption, though. They're intended to serve as a sort of institutional memory for the unit, as well as produce material to sell the work of the agency to the current administration. They may also produce 'official history' type manuscripts, but that's often not a primary job responsibility. There are plenty of these histories that are pretty high quality, in any event. Examples:

http://www.amazon.com/Secret-History-Classified-Operations-Guatemala/dp/0804733112 http://energy.gov/sites/prod/files/Summary_History.pdf

Relevant article: http://www.eenews.net/greenwire/stories/1059997283/print

There's probably some bias imposed by the circumstances, but then again everyone works from some perspective or another. I can't imagine an official agency historian being anything but upfront about that.

A bigger problem with military agencies' historians is probably that military history is currently something of an inbred field that's more than a little influenced by disconnect from the rest of the discipline through institutional issues like many of the positions being at service academies and other military-sponsored sites where there isn't necessarily the broader intellectual community. That's not 100% the case, obviously, but I'd suspect it imposes more bias on the questions that get asked, type of approaches to answering them, and quality of research than happens to official agency historians.

I have the most experience with the US Army Green Books, and other official military histories. Actually, you would expect most of these books to be super patriotic and chest thumping, but (especially in the case of the Green Books), these Official Histories tend to focus more on an accurate retelling, than an "politically correct" retelling. These books werent written as a history for historians, per say, but to describe (in bone crushing detail) what happened and why in the war. These OHs then become important learning and analytical tools for future generations, who are able to study an impartial account of the successes and failures of majors wars. In this regard, most militaries are actually pretty honest. Even the Soviets, when they wrote their own official histories, were pretty clear about when and why the Red Army failed. These officers were more or less honest about events, as it helped teach future generations what to do, and what not to do, in similar circumstances. As a historian, if somebody wrote a book (say on the US War effort in WW2), and they didnt use the Green Books, I would be a little suspicious as to the quality of their research. They are fundamental texts for academically understanding wars.

This isnt to say there is no debate past the OHs. They really lack the interpretive element that comes from pulling multiple sources together, as well as the relevant primary documents, but they are a critical starting point for research.