Aliens use to be hands down my favorite cam for big walls and climbs where you have tight and shallow placements. They find their way into more placements than almost any other small cam. Then, in the last 5-10 years there have been a lot of new cams introduced that give the Alien stiff competition: Metolius Master Cam and Black Diamond Camalot X4. The main downsides to Aliens are their reliability, durability and they are hard to find.
NOTE: This is a review of the original Alien which is no longer available. The Fixe Alien has replaced it. Read more about the Fixe Faders Alien Cams