These things. Where to start on these things that hasn't already been said. Well, beyond make peace with your squad if you ever have to do a base assault and don't bring a durable tank to scout for these.
Awesome picture. It makes me remember the first time I met a chrysalid, or Alien-look-a-like as I referred to it as then.
It came charging at my troops from out of nowhere and just barely managed to poke one of my guys before going down. "Okay, so it was horribly hard to kill, but now it's dead and dealing with the zombie should be easy, right? Shoot the zombie a bit, la deh dah, okay it's dea-OH GOD DID IT JUST TURN INTO ANOTHER ONE OF THOSE ALIEN-LOOK-A-LIKES?! WHO ELSE CAN SHOOT THIS TURN? NO ONE? FFFFFFUUUUUUUU---!"
In the end I went for just trying to grab a corpse and running for the plane to get one to study. The one last soldier running made it to the top of the ramp, one step away from safety before running out of time units. Cue classic Alien scene of being tackled into the plane by the monster and banging noises followed by blood dripping out.