Came across a very positive retrospective of Marathon Infinity dating from 2007.
It starts out...
If Marathon was an earnest indie film and its sequel was the crown-pleasing Hollywood smash, the conclusion of the trilogy -- Marathon: Infinity -- was a daring arthouse creation designed to challenge and mystify its audience.
Great read... you can find it at
http://www.1up.com/do/blogEntry?bId=8380021&publicUserId=5655917
From that page you will also be able to find retrospectives of Marathon and Marathon 2.
Cheers
Hamish