I didn't buy the first Fantastic Four Omnibus when it was initially released. I already owned the Masterworks editions, and I didn't see the point. But when this volume came out, no matter how hard I tried, I just couldn't ignore the ratio of greatness included between the two oversized, hard covers. True, I already owned the Masterworks that included this same material, but I sold those suckers once I got this, covered the cost of this beast and made a bit of a profit. But before I sold those Masterworks, I compared the printing in each, and the Omnibus BLOWS THE MASTERWORKS AWAY.
I was especially impressed with the way they corrected the printing of Fantastic Four #52, the first appearance of the Black Panther. In the Masterworks edition, Volume 6, I believe, the Black Panther is just a blob of blackness. In the Omnibus, you can actually see the interior lines and the shading. In that same issue, the Fantastic Four look drawn with cheap ballpoint pen if the Masterworks are to be believed, but the Omnibus returns the proper line weight and matches the original art with much more accuracy.
If you think you already own these stories (and by the way, this Omnibus reprints Fantastic Four #31-60, plus Annuals #2-4), and you don't own this volume, then you're wrong. The Essentials, the Masterworks, even the original comics, can't compare to the way the stories look here. And, as you must know, this is the height of Lee and Kirby: The first appearance of Silver Surfer and Galactus, the Frightful Four, the Inhumans, Black Panther, "This Man, This Monster"--these stories are all in one gigantic, beautiful hardcover edition.
The only reason not to buy it is the cost. But, you know what, you're worth it.