This depends a great deal on both which branch of philosophy we're talking about & who is evaluating that particular branch's usefulness.

For example, I find developments in logics, philosophy of science, & general epistemology to be of great interest, and I perceive all three topics to be advancing (listed in order of priority as that goes) as the years go by. I'm sure others feel differently.

It would be hard to get past the fact that, especially between the different branches of philosophy, there is a great deal of "philosophy of language" that is or must be done just to get at what anyone's talking about. But that is, to some extent, true of any field with a technical language.

So I guess all four answers make sense in some sense.