Grande Armée will boost your military units' training speed significantly. It can be very useful for those aiming for a Domination Victory to start prepping up for a late-game war to overthrow the last few civilizations that remain after the early-game conquests. (And if you haven't done some early game conquering, why are you still aiming for a Domination Victory?)
However, civilizations reacting to a war declared by a stronger enemy could also use this policy effectively, and come up with an army where before there were only easy pickings for the enemy.
Note that the Production bonus does not apply to siege units! So if you are the defending side and suddenly you are now counter-attacking, this policy will not be useful for training machines to take down enemy walls.
At its peak just prior to the invasion of Russia, Napoleon’s Grande Armée numbered some 680 thousand men, drawn primarily from France but also including contingents from 11 other nations (95 thousand from Poland alone). It has been widely acknowledged by military historians to be one of the greatest fighting forces ever assembled, and under the command of Napoleon and his generals, tactically superior to anything it faced. The Grande Armée was originally the force Napoleon had assembled for the invasion of England but renamed in 1805 AD and put to other uses – like conquering most of continental Europe. It would serve as the template when most of the powers decided they too needed a large, national army.