I may be what Chris Day (author of the essay that bornagainanarchist has linked) is calling an anarchist who has taken anarchist propaganda too seriously, but it seems to me that the anarchists had no problems forming an effective set of autonomous militias in Spain or in Ukraine.
A problem they did have in Spain was that they made the mistake of forming a popular front with the communists and social democrats, who then ordered them to accept the centralized, governmental command structure and military discipline, or else to disarm. These events are described by one of these uncontrollables in "A Day Mournful and Overcast." The uncontrollable also describes what the anarchist militias were like in terms of how they elected their leaders and coordinated with each other.
Orwell (not exactly an anarchist propagandist) also recounts some of the relevant communist-anarchist conflicts in his Homage to Catalonia, if I recall correctly, and likewise Enrico Arrigoni in his field reports included in his autobiography Freedom: My Dream.
Modern combat theory, from the little I know of it, definitely considers guerrilla armies the most effective form of organization in asymmetric conflict against a more powerful opponent. And there are plenty of examples to point to here. So really the question seems ideologically motivated. People who want to make a case against anarchism might point to armies -- aha, armies involve hierarchy and authority, gotcha! But it just doesn't seem to hold any water. Anarchists abandoning their principles, such as to join a government in Spain or to join a popular front, has caused more ill than anarchists militias lacking heavily centralized command structures.
I can't speak to the economic question as I don't take any interest in such things, but for the sake of the site organization you should probably ask it separately, as it is a separate question.
(punctuation edit for clarity)
