You can still pull off a good adaption, a scanner darkly and blade runner (two Phillip K. DĀick adaptions though Blade Runners title is "Do androids dream of electric sheep") are two good examples, though blade runner is good considering the effects back then and the stylistic nature that was around during that time, reminded me of Nikkita. But there are obvious flops, lord of the rings while a good movie, its such a long book that to adapt it is hell aha. Especially before they even get started like everything Pre-Tom Bombadillo is about half the book...
Ah now only someone who's not read Jurassic Park would say that. Jurassic park is more complex than Angel's and Demon's (Book-wise). Granted it might suit dumbing down for the masses/film conversion more than Angels and Demons does but it proves it can be done. Jurassic Park book-wise is extremely detailed and technical, but they managed to once again leave out masses of stuff (and actually, change the whole story, for example, Hammond, one of the MAJOR character's, dies) and still make one of the best selling films of all time. Angel's and Demon's didn't pull it off.