However, when I heard that WotC was not only reissuing the original A1-4 "Slavers" adventures, but also including a brand-new A0 prequel module, I had to get the book, if only for the sake of completeness.
I have to say I'm pretty impressed.
The new adventure, written by Skip Williams, is A0 "Danger at Darkshelf Quarry". It takes up 23 of the first 28 pages of the book, and seems to be a suitable way to easy the PCs into the machinations of the Slave Lords (although it takes place in southernmost Nyrond, as opposed to the Wild Coast where the other adventures in the series take place). It's designed for characters level 1-3, but given that Slave Pits of the Undercity is for levels 4-7, and I don't really see any way the adventure could jump up PCs 3 or 4 levels, some more intermediate action is probably going to have to be invented by the DM to bridge the gap. One thing I really like is that there's a mocked-up module cover for Darkshelf Quarry on the back card of the book, done up in a similar style to the other four module covers. That was a nice touch.
The back cover card does indicate that the adventures are "Playable with D&D Next rules found at DNDNext.com". However, there are no mechanics for the new version of the game in the book; to use it with the new rules one must download a conversion packet from the website. Even the new adventure is written using the 1st edition rules. I didn't see any module conversion notes in my copy of the latest playtest packet, but I did see a bestiary with D&D Next stats for the various A0-4 monsters (well, A0-5; apparently this month's Dungeon magazine will feature another new adventure, "The Last Slave Lord").
On the whole, I'm very pleased with this book, and will probably want to integrate Darkshelf Quarry into my next Greyhawk campaign.