I think it's clear that, in the case of Darwin's Finches, we have a situation of an adaptive radiation with speciation in its very early stages. Hybridization still occurs, with about 50% of the species in the archipelago, but it is always at a very low frequency. Most of the populations of finches are reproductively isolated pre-zygotically: they mate only with birds that either look like their parents in color and size, or sing the same song as their fathers. This effectively restricts gene flow between them. At the very least, it allows for morphological differentiation to occur, which reinforces the behavioral isolation even more. However, they haven't been isolated long enough for much post-zygotic isolation to develop. In addition, the situation there is very complex, with colonization and recolonization having gone on for some time.
So, overall, I'd say Darwin's Finches are an excellent example of adaptive radiation and a wonderful example of the beginnings of speciation.
KC
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