So I have actually seen all 5 nominees in this Oscar category and will now pontificate about who will win and who ought to win. You're welcome.
First, the nominees: District 9; An Education; In the Loop; Precious: Based on the Novel 'Push' by Sapphire; Up in the Air.
Second, I haven't read any of the source material involved here. I doubt most Academy voters have either. They are probably voting on the screenplay itself without regard to how difficult the adaptation was, or how true they were to the spirit of the source material.
Up in the Air is the likely winner here, in part as a consolation prize for director Jason Reitman. Reitman's made 3 solid movies in a row (his first 3, by the way: Thank You For Smoking, Juno, and this). It's a smart and funny movie, and it seems to have connected surprisingly strongly with critics picking up a variety of awards from top critics' organizations. I don't see a strong swell of support for either of the potential spoilers (Precious and An Education), so I'm pretty resigned that this will win. I'm okay with it winning, but I wish more attention were being paid to the screenplays for Precious and An Education.
Precious has gotten good reviews and Mo'Nique is all but guaranteed an Oscar (Supporting Actress is another category where I'm done with all the nominees, so I will write about that soon), but Lee Daniels, the director, put together a fantastic movie and the screenplay has moments of sweetness and inspiration. But it's dark and somber at times as well. If Slumdog Millionaire hadn't pulled off the complex mix of emotions so amazingly well the year before, I think people would be reacting more favorably here.
An Education only picked up three nominations, but the more I talk about it to others, the more I realize how much I adored the movie. Its so artfully, subtly put together, and filled with top-notch acting, that it reminds me of a Merchant-Ivory film. The screenplay has less emotional oomph than Precious, but the dialog is every bit as smart as Up in the Air, and the movie's twists unfold so nicely that I was taken completely by surprise (which is all I will say there). Director Lone Scherfig is clearly a talent and has a deft hand with actors--here's hoping we see her live up to the promise of this movie in the future.
So if I were to rank them based on my assessment of the screenplays, I'd put An Education at the top, followed by Precious, Up in the Air, In the Loop (which is wickedly funny, but not much happens), and then District 9.
As a post script I will say that this is a solid slate of nominees. Even District 9 has a lot of merit. The only movie I could imagine being here in lieu of D9 or ITL is Star Trek, and that had red matter, and the whole lack of OSHA for Romulans issue (its brilliance is in the exuberance of the direction and the fantastic cast).