In some of the larger sizes, I think Aliens are totally replaced by Totem Cams, which are now cheaper as well.
Purple Totem approx = red Alien
Yellow Totem approx = grey Alien
Blue Totem approx = yellow Alien
(sizing by memory here; I don't have my Aliens handy for comparison.)
The Totems have greater holding power, heads as small or smaller than the Aliens, are just as flexible, and offer the possibility of placements involving just two cams in desperate situations.
The green Alien is still an important Gunks piece that doesn't seem to me to be duplicated by anyone else. Smaller than that and I think C3's are a better bet.