There is no objective way in which to answer this. When it comes to systems of government, goodness or badness is a subjective quality. We Americans typically believe in liberalism/democracy and therefore we think that military rule is a bad thing. However, one can argue that it is not really so bad. It is also true that there can be different types of military rule, some of which would be more acceptable to us than others.
We Americans believe in democracy. We believe in being able to vote for our leaders. This is something that is not allowed in a military government. Therefore, we think that military government is a bad system.
We Americans also believe in individual rights and freedoms. Most military governments do not allow as many freedoms as we do. Often, military governments are rather repressive and allow little in the way of freedom of speech and other such fundamental rights. To Americans, military rule would be more acceptable if it protected rights than if it did not.
However, it is possible to argue that a military government is a good thing. If civilian governments become corrupt or abusive, you can argue that a military government would be better. This, for example, is why many people in Egypt right now appear to prefer the military government to the previous government which was democratically elected.
We cannot objectively say whether military rule is good or bad. We Americans would approve of some military governments more than others but, on the whole we would not approve of military government because those governments are not subject to being elected by the people.