Ok, I've read the first 4 volumes of the manga and I'm kind of disappointed. Sure the manga added a little bit more back story, but not enough to make up for the lack of story.
Mainly: didn't have the Barry the chopper back story, didn't have the Slicer brothers wanting to die and one of them killing themselves (adds more to how Al is feeling), Had Tucker dying?! Wtf, didn't have the fake Elric Brothers (I think they were pretty early in the Anime), didn't show the scene of Greed becoming free when the 5th Lab fell, didn't show Envy "coming back to life" as the priest and showing everyone that he was alive and turning the dead birds back to life, didn't show Rose's boyfriend in the first episode, didn't have Ed talking with General Grahn as he died, didn't have Scar coming to "save" Al from Barry's armor, Scar seems much more evil and almost diabolical in the manga, I'm probably missing more but you get my point. Also, keep in mind that I'm only half way through Volume 4 and it has this many changes
Now maybe some of the things I listed were for filler, but they still added much more to the story.