Drinking the Sleg makes you damn near invincible, so you can get thru anything. It's not really a bad ending that it does.... it's just that there is a 100 point achievement for completing the game without drinking any at all! "Above the Influence" is the name of it.
I'm in my second play thru now... the first time I drank it without realizing I really had an option! So second time, I'm playing with the strat guide, and I now know I do have a choice! =o)
I don't know about the crucible yet, tho... will do that after I'm done here.