The first trilogy is compelling...lots of action and interesting speculation on how technology will affect human society.

Moving ahead 20 years, we find our Marines fighting a new set of earth-based enemies and getting a glimpse of some extra-terrestrial ones...

Of the first three books, I found this to be the the least compelling, but most important since it introduces us to an external threat: the so-called "Hunters of the Dawn" and sets the stage for the next trilogy where our Marines find themselves confronting alien threats among the stars.