Very nice game. I liked the progression and the variety of endings.
For me, the most difficult ending to achieve was "The Red Trail" (5th mutation, no "safe room") which required more strategy in the final fight than the other routes. Only after I finally completed all the other endings and learned to focus on my strengths, did I manage to get it.
There was funny moment when I realized I know the place in the "Underground Facility" picture (Turda salt mine, in Romania) :)