I just finished the game for the second time on survivor mode rescuing all survivors and replaying the game was just as fun as playing it the first time.
Now I wanted to say that this has become one of my favorite games period on the 360. Even though this is a survival adventure game, I Am Alive does survival horror aspects extremely well with the conservation of items/ammo, the grainy, dusty setting atmosphere and the perfect music to build up suspense or combat music. Also, the feeling of desolation and periods of silence. The innovative climbing and combat system is refreshingly challenging, which is a welcome change from games nowadays that have to hold your hand every five steps. Overall, this is a fantastic game for XBLA and I would recommend it over many full priced retail games and it even shines among survival horror this gen which has been really disappointing overall (Resident Evil 5, Silent Hill 5, etc..).
I would say the main reason for all the negative reviews from critics and other gamers is that the game is very challenging with the checkpoint system and some trial/error gameplay, which is frustrating for many who are not used to it (luckily I recently finished Call of Cthulhu which is way more frustrating than this game). There were games like this in previous gens but very few now.
I would love to see a sequel to I Am Alive in some form whether a full priced game or another XBLA title.