In general, contemporary philosophers don't accept Kant's account of understanding. There are at least two different strains at work in rejecting Kant's categories of the understanding.
Stated in the simplest and roughest terms, Kant believes that we understand objects through the application of the categories of the understanding to perceptions shaped by through the forms of sensibility as applied to things. For Kant, things are in themselves in accessible to our understanding and imperceptible. To access these things through our senses we must subject them to the forms of sensibility and to understand them we must apply the categories to them.
There's a question about the relationship of mind and reality that the Kantian picture assumes, and this creates two directions of objections. First, you can believe that we have more direct access to things (and reject this three-level distinction of what we are encountering). This sort of account is prevalent in contemporary analytic philosophy. Understanding is seen as not occurring in such clear distinction to perception. Thus, we deny the need for a Transcendental Unity of Apperception to glue everything together.
Second, you can argue that what we do is not grounded in the thing-sensible-object system. In other words, we have objects but they need not be so tightly linked to reality. Part of the reason is the recognition that Kant's 12 categories are kind of arbitrary (despite their systematic brilliance). On these accounts, offered say for instance by Rorty but having their genesis in Hegel, our access to reality is seen as limited by our modes of understanding. Depending on the species this can be pernicious to the point that there's knowledge -- just interpretation or softened such that we still have access to things through the objects but according to different lists of categories.