The IGN one seems to be the lowest, looking at Metacritic across reviewers you've got high 70's/80's.
There is a pretty useful breakdown on CVG
http://www.computerandvideogames.com/478130/alien-isolation-review-round-up/
I think if you're just a general game fan, with no specific interests in Aliens, then I suppose you have to just weigh positives and negatives.
If you're an Alien nut like myself however there looks like more than enough in the reviews that have come so far to make this an essential purchase.
Actually, this is the excuse I've been waiting for to get myself a PS4 - can imagine this with a decent pair of headphones in a darkened room being bloody awesome.