How reliable is knowledge gained through introspection? On the one hand, we have been learning that knowledge gained through introspection is surprisingly fallible. A philosopher who has worked on bringing this fact to light is Eric Schwitzgebel. In this paper, among others, he has argued that,based on the evidence, knowledge gained from introspection is less reliable than knowledge gained in other ways. He now has a book out called Perplexities of Consciousness which looks worth reading -- see reviews here and here.
On the other hand, perhaps there is still something special about knowledge gained via introspection. Schwitzgebel participated in a debate on introspection with Brie Gertler last fall on Philosophy TV. In a draft of a forthcoming paper called "Renewed Acquaintance" (Word doc), Gertler defends a view called the acquaintance approach to introspective knowledge of phenomenal qualities (a book is forthcoming titled Self-Knowledge).
Gertler's view, which is inspired by Russell's theory of acquaintance (see here for some history), claims that, in at least some cases, introspective knowledge of the phenomenal is direct, as the conscious state is itself the basis of our knowledge.
There is a sense in which this seems a radical view: the direct grasp of the phenomenal constitutes (as she says) an intersection between reality and the epistemic. On the other hand, however, the view is modest in the scope of its claims: Gertler is not claiming any widespread infallibility about introspective knowledge, even of phenomenal states. Only some introspective knowledge is knowledge by acquaintance -- namely very stripped-down examples of attending to a simple experiential quality. Further, even these judgments need not be claimed to be infallible, although it is claimed that these judgments are better justified than normal empirical judgments. So Gertler is able to agree with Schwitzgebel and others when they point out the variety of mistakes we are prone to make (my own view is that the only infallible judgment is probably a "meta" judgment that we are indeed having experiences featuring phenomenal qualities). As a result, I find Gertler's case is pretty strong.