In my experience, it has always been sarcastic. As Partridge notes, it means that one has all the “good” qualities of a white person. (Much as Huck Finn described Jim as “white inside.”) However, the whole notion that white people have inherent qualities or virtues superior to blacks (or other non-whites) is so silly that it is generally used as a parody of some stereotypical racist who would think that it actually was a compliment.
YMMV, I suppose.