It should follow original plot but add own stuff aswell. It's good if they follow original plot but it doesn't mean they can't add own stuff and take their own take of something. Trough they should be really big, expecially taking own take of something.
I'd say to follow the story but also have a few new ideas as well, that's why movies and books are the same yet different. If they follow the story exactly then we (if read the story before hand) would already know what is happening or going to happen before it happens which would make it boring and a waste of time. Unless people would like to see it happening in motion picture then through imagination of words.
I agree with a few others, it really does depend. Some VG's or comics just don't have any room for anymore ideas in the story lien without messing it up. For others, it might be better for the movie. But for all of them, the basics need to be absolutely firm, no maybes when it comes down to a comparison of the basic structure of the plot. Otherwise it isn't really the movie of the storyline at all... unless, of course, it is just relating to the same world and is a different installment. but then again, it wouldn't make much sense unless you were a previous fan, and, well, with the way people are, that wouldn't help the fandom as much.... GAH, I'm making no sense. xDD