Sorry to blab the obvious, but this has to be said, frequently. It's one of the best scenarios for the Aleph One engine that I've ever played, and I rate some of those very highly. I'm having a blast playing through some of the levels even though I played similar ones a few months ago from an earlier version. The balance between playability, intrigue and plot and the fun of wiping out hordes of aliens is set very well. The way the story plays out is nice, and it enhances the gameplay without getting in the way of the game action.
I'll comment more when I've played through the whole of the current version, but even the version with two placeholder levels kept me interested for weeks.