I think I've watched enough of the TV version of FlashForward to have an informed opinion. There are some spoilers in this post, so be forewarned.
They've kept several elements of the novel. There's a world-wide flashforward (duh); they set up a website for people to post and compare their flashforwards; and one of the main characters sees only darkness in his vision, and concludes that he's dead in the future.
They did make some major changes, and not all for the best. One is that the flashforward is to events only six months into the instead of decades as in the novel. That makes sense, since it means the characters can discover if their visions are valid or not within the arc of a season or so. Plus we just learned that this was not the first flashforward, which means we might see the protagonists having new visions in upcoming episodes.
Something else they're doing differently is that the flashforward is no longer caused by an experiment at the Large Hadron Collider that releases energies not seen since the Big Bang. Instead it seems to be the work of nefarious individuals, some of whom were conscious during the event. It's a twist with plenty promise for the show's plot, although it's not nearly as plausible as the original premise.
The other big change is the characters. Lloyd Simcoe is still there, but as a secondary character. Theo Procopides is out, his role now taken by the character Demetri Noh, who is engaged to be married. Michiko is totally gone, replaced by Dr. Olivia Benford, wife (for now) of the lead. As for the lead himself, he's now FBI Special Agent Mark Benford, a recovering alcoholic who has a vision of a pinboard covered in clues about the flashforward event.
This brings us to the stupidest change they've made. The main characters are no longer scientists, they're FBI agents. This has to be the most hackneyed and unoriginal choice possible. It's a case of the show's creators blindly aping X-Files and Fringe instead of doing something original. And the idea that the FBI would be in charge of investigating the biggest and strangest scientific event in human history is ridiculous. It's as silly as putting the Mounties in charge.
Plus there are some changes that should have been made that weren't. When the novel was written back in the 90's it made sense to set up a single website for people to report their flashforwards, but now that we have blogs and twitter isn't the whole idea redundant?
Anyway, overall the show is doing justice to the novel. There's a bit too much pseudo-religious cant about "purpose" and "faith" (which relies on the Fallacy of Equivocation, as you can imagine) but that's to be expected from a show trying so hard to continue the Lost phenomena that they even included a billboard advertising Oceanic Airlines in the background of one scene.
Whether or not the show delivers on the promise to be the new Lost remains to be seem, but it is off to a good start.