In the opening of Beyond Good and Evil, Nietzsche attacks Kant's critical philosophy, picking out the question "how are synthetic cognitions a priori possible,' but Nietzsche fails to give an explanation of what Kant was getting at with this question. The question is actually an attack by Kant himself against traditional metaphysics. It reflects Kant's demand for the kind of a priori certainty found in mathematics or natural philosophy (physics)--which is why he undertook his investigations into those fields. Kant wanted the same for metaphysics: judgments that are universally, and objectively valid; as opposed to judgments (like the traditional systems of metaphysics) founded on a posteriori reasoning. Nietzsche likely would have launched his attack regardless, even if he had understood Kant, but it would have helped further his argument against what he regards the pretentious (childish) aims of metaphysicians, if he had a better grasp of Kant. Kant's response to Nietzsche would be that Nietzsche launches an attack against something that does not yet exist. As Kant states in his critical philosophy: "It [metaphysics] cannot exist unless the demands here stated on which its possibility depends be satisfied; and, as this has never been done, that here is, as yet, no such thing as metaphysics." Nietzsche, in other words, is throwing his punches at a ghost. That might be an appropriate description of metaphysics from the point of view of those who have no use for it. But it's all too easy to attack something if the questions it seeks rational answers for are thought to be beyond the sphere of what can be humanly understood. This is a skeptical, negative mindset however, and an understandable one; but the attack of Nietzsche against Kant sounds more like a not too well orchestrated rant rather than a methodical, well rounded and justifiable critique. Nietzsche's style is more akin to throwing everything that one can grab at the wall to see what sticks. The result is a hodgepodge of ideas that are confused together and made into a tasteless, ill-thought out stew.
Just the sort of thing anti-philosophers and anti-metaphysicians like to chew on.