The Explication rules are great and there's definitely lots of super cool stuff to play with. This book is jammed full of stuff. There's allusions to nids all over the place (swords made from scything talons, Vouxian Stalkers/Shades, etc.)
As mentioned earlier, not a whole lot in the way of new aircraft despite introducing an aircraft-focused role.
Keep in mind that the Ace is not purely aircraft-focused. It's a piloting/driving/riding focus. They also happen to make some pretty cool tech gun fighters when combined with the right homeworlds. Disappointing that there's no gun-cutters (ala Eisenhorn) or Valkyries though, for sure.
Probably because those are able to operate outside the hive's mind synaptic web and there is no Tyranid fleet in the Askellon Sector (yet).
And I'm totally ok with this. The two profiles are really deadly, scary creatures perfect of Dark Heresy. If you get to the point of massed planetary invasions with gaunts, carnifexes, etc. it's time for Deathwatch anyways.
All in all, a really great book.