I am a big fan of Undermountain and Greyhawk Ruins, especially to former. It's like the matterhorn of dungeons. It's almost more a setting than a dungeon, and I have done it a lot in the past.
At the same time, as I have grown older, I find I have less patience with big dungeons, and time constraints keep me from using them regularly. Heck, we spent like four sessions in the first of three adventures in a smallish module by necromancer games (Demons & Devils); by the time we were done, I was tired of it.
Undermountain was sort of made to let you set other adventures in. You do have a little leeway in that you can use gates to get your PCs in and out, but it always struck me as a bit tacked on and a bit of blatant fiat. If I were to entertain another dungeon as big as undermountain, I would like to have similar convenience to insert and extract players, but better explained and less "handwaving" in nature.
The ad copy brags that it has more encounter locations than undermountain. Okay, but undermountain's locations are detailed (often spanning multiple pages) and many are interesting. If the "encounters" in this books are a stat block and a paragraph of why they are in the room, that is not really saying much. And somehow, I doubt that many encounters can be detailed to the same level. (I think I'd have to do a bit of quick math, but a quick BOTE: IIRC, the undermountain core book is 160 pages. So ~1 encounter per page if their figures are accurate. Compare to 10x as many encounter in 5x as many pages. And also consider stat blocks are more detailed now. That leads me to beleive that the actual content won't be as high quality.
Anyways, there still might be something worthwhile there. The promises of conflict built into the backstory sound useful.
I guess I'd really want to know: who is writing this? Some big names might persuade me that it is likely worthwhile more than others. But then, I don't see many big names spending this much time on such a big project.