[Sheep]

Well for me it depends on the series if they stick to the manga or not.

Like take the re-make of Negima! (the one with the ? at the end)

That didn't follow the manga at all, in fact they had to make a spin off manga for it. I thought it was one of the best copy's of the Negima series in anime form. Then again that series gave more screen time to the small characters in the class, and still used the same designs and characters from the manga. (even if some hair and eye colors where changed)

Or take Umineko if you want a more rage induced answer

They tried to keep all the important scenes in that, that kind of failed when they gave almost all of the 4th arc's screen time to pointless scenes that could have been done in 10's mins tops not the full episode. Plus they kind of ruined how some of the characters act because they cut all the scenes with development for them.

Still some copy's of a anime turn out better then the manga or light novel it's pasted off of. I for one loved the Spice and Wolf anime, and when I read the novel I kind of thought it worked better as a anime.