I beat it in Casual in about 2 days. Hardcore in about 1 day (since I knew where most things were). Insane took about 2 days to beat - those damn glowing wretches were a PITA! They definitely needed to add a couple more levels to this game.
I agree with having a real person as your squadmate as well. Was it just me or is Dom the biggest idiot in the world? lol