AUTS is an early game in the genre, and a very important one. If not the actual first to bring the caveflying/shooting v-shaped ships battling against the gravity on PC, then AUTS definitely is the first in the genre to do it just right, and all it's (spiritual) descendants owe a great deal of their general playability to it. AUTS still stands out for being quite difficult and unforgiving, for example colliding to a wall eats up a lot of ship's energy, which you really can't afford to waste. There are also a few special weapons which are still unique (for instance the powerful Sonic Boom or the annoying Dirt Clod), but besides Fan, which can be over-powerful even in relatively unskilled hands, they don't overthrow the pellet gun, which remains the deadliest weapon in skilled hands. Thus, AUTS is a true test of marksmanship and competent piloting.
Note: The bait for registration used to be a bunch of levels and the latest version, so although the game says "unregistered" in the bottom of the setup screen, it's not crippled in any way.