Paul Jenkins´ Inhumans is their best book. But the
War of Kings event is also pretty good.
Aside from that, the Inhumans are usually pretty good supporting characters. They are interesting because they are aliens, weird, isolated, mysterious and so on.
Paul Jenkins´ run is good because it is a mini-series that keeps that, you spotlight them, but keep them aloof, weird and unexplainable enough.
When they turn into the X-Men, the concept basically died.
I have hope that series, ironically, will revitalize them; that will bump them down to this weird secret eugenistic alien society again.