The Stanford entry on Functionalism

Here. 

The final paragraph is as follows:

In general, the sophistication of functionalist theories has increased since their introduction, but so has the sophistication of the objections to functionalism, especially to functionalist accounts of mental causation (section 5.2), introspective knowledge (Section 5.3), and the qualitative character of experiential states (Section 5.5). For those unconvinced of the plausibility of dualism, however, and unwilling to restrict mental states to creatures physically like ourselves, the initial attractions of functionalism remain. The primary challenge for future functionalists, therefore, will be to meet these objections to the doctrine, either by articulating a functionalist theory in increasingly convincing detail, or by showing how the intuitions that fuel these objections can be explained away.