From what I understand, there were two-disk deluxe versions of Heaven and Hell, Mob Rules, Born Again, Seventh Star, Eternal Idol and Dehumanizer. I didn't know Forbidden was next (I figured they may go chronologically) but in my book, they are must haves. Compared to Ozzy/Sharon (who left off stuff, like "You Said It All", from the Blizzard deluxe versions), they've been pretty thorough and complete in putting b-sides and stuff on there.
I haven't done a detailed comparison of the sound, but generally they sound pretty good (Born Again is a sure improvement over previous releases).