The extra content is decent but not amazing, I think it was included in an earlier hardcover printing, the Graphitti addition. It's not as great as it could have been if they could have actually gotten Moore involved, but that wasn't gonna happen.
I think there are some script excerpts, pinups/promotional art by Gibbons, and um, I can't remember what else. It's not exactly jam packed but it's stuff that wasn't in the SC version I have. There are 48 pages apparently.
Here's a description I found: "DC has reprinted everything that was included on the Graphitti Designs edition that has been out of print for years. Included are Alan Moore’s and Dave Gibbons thoughts and afterwords on producing the series, a sampling of the scripts, background information on every character and just about every promotional piece of art produced for this series, including overseas art and the portfolio album art."
I'm not sure I'd buy the book for the extras, but for I would for giant sized cleaned up Watchmen WITH extras
I can take some pics tonight if you want.