Intrepid modder TemplarGFX has overhauled Aliens: Colonial Marines, taking it from middling shooter and legendary disappointment to competent FPS that does some justice to its source material (I fear the disappointment might be permanent).
TemplarGFX's ACM Overhaul, on ModDB, focuses on rejigging the xenomorph AI to make them a credible threat. Headshots are essential, acid blood is a constant worry, and aliens have had their decision timing increased by at least 500%. They move and accelerate faster, and every animation has been re-timed to make them look more organic.
Human animations have received the same treatment, and even the voice work is better timed to provide better situational information. AI vision has been reworked too, offering the potential for rudimentary stealth.
Accompanying the rebalance is a suite of graphical enhancements: brand new particle effects, higher shadow resolution, more dynamic shadows and permanent gore and clutter.
The full list of changes is as long as it is impressive, so if you have it in your heart to give Colonial Marines another go, ACM Overhaul is as close as it's likely to get to the sales pitch.