Underrated. Probably not as good as Fury3, but give it a break.
But let's speak about the game itself.
This game is a sequel to Hellbender and Terminal Velocity. you're Councilor, the only qualified pilot skilled enough to fly the Hellbender combat aircraft. The bions have returned (to take another beating), and you must destroy them... again.
The flying physics are pretty much the same as Fury3. You can do Barrel Rolls, make yourself upside down, etc. The mechanics are great, and it makes it satisfying to fly the Hellbender. The missions are similiar to Fury3's: you go to a planet, do around 3 missions, and then go to another. Well, I guess you already understood that the overall design is identical to Fury3's. You can fly around the planets and go off their atmosphere, explore caves, kill some enemies, etc.
The camera is decent. Again, it's identical to Fury3. But the best part of all camera options is definitly the cockpit, it's amazingly well done, making you feel like you ARE IN the Hellbender.
The missions are challenging and make you think before you leap. It follows it's motto "Shoot to kill. Think to win."
I can't review the multiplayer mode, has I never tried it against other players. Although, I already played multiplayer alone (don't ask), and it doesn't look that good. The music's boring and there are only THREE maps.
But overall, the game looks like a blatant expansion pack to Fury3, there's no real changes except for the story and the missions, even the WEAPONS are the same! Almost nothing has changed! Although, the graphics improved a little, and it's still a satisfying experience, even to this day.