There's no demo, just the promotional video.
Creating a playable demo for something like FOF would take A LOT of programming time, delaying other projects significantly. And since the game has been out for nearly two years it simply wouldn't make good business sense to produce on at this point. So, the video, reviews, AAR's, and, of course, forum discussions will remain the best way of getting a sense of it.
Michael Jordan plays ball. Charles Manson kills people. I torment eager potential customers by not sharing screenshots of "Brother Against Brother." Everyone has a talent.