Fun, massive, and gratifying... albeit a bit drawn out and too long...
I felt the middle and late story elements were bogus: they lost much semblance of story/plot, and were not very motivating-- I honestly just wanted to finish the game. I think there were at least three that went like this:
"Prophet, come find me, I'm at the North tip of the Island!"
*30 minutes of exploring and combat later...*
"Prophet, I've escaped to the South tip of the island, come find me!"
*30 minutes of exploring and combat later..."
Repeat.
The end dragged on too long... too many alien fights... the same 4 enemy types... TOO MANY **** PINGERS. One was enough.