H.E.D.Z.: Heads Extreme Destruction Zone

This game is very old and wasn't hugely well-known, but I hope some people remember it! Basically you play as an alien that goes around fighting other aliens, all the while the aliens are all wearing "heads". These heads are effectively masks from abducted humans etc that the aliens have produced and allow the wearer to use the powers of whatever they extracted the head from. It's much cooler than I made it sound though don't worry. :D

So did anyone play this game? What was your favourite head? Did you collect 100% of the heads? Which was the hardest secret head to get? Did you find the game difficult? Was your game full of glitches? Do you now despise Isaac Newton? Discuss!

I play it everyday; first played the demo back in 1999, but didn't get the full version until 2010.

Favorite Head: Animation Hero (gotta love those Airstrike heads)
Collected 100% heads: Yes
Hardest secret head to obtain: British Army Private (found in Jungle Base by activating a switch and using a "flying" type head to reach the hidden area the switch activated.) I also think that head is supposed to be based off of Adolf Hitler, or Saddam Hussein, but I'm not entirely sure on that.
Certain parts of the game were difficult, mostly with all of the AI attacking you, while some of their attacks seemed to insta-hit you/travel toward you too quickly.
Discoverer of Gravity (Isaac Newton) is supposed to be an Airstrike head, but he just makes your game give up on life whenever you use his attack. I had trouble getting through Jungle Base because of him.

I also just joined this community just to respond to this thread. It's a shame it's not well-known; it's pretty fun for a classic, obscure PC game. You don't get this kind of thing often.
I'm actually uploading a walkthrough of it on YouTube (again, since I did it two years ago and removed my account a few months after), so go and look for it if you're interested. It's nice to know some people still know this game exists. Like I said, I still play it everyday if I have time.