Looks good
So far it looks pretty solid. As you'd expect, it's half targeted at PCs and half targeted at DMs.
The PC half has all the stuff you'd expect - possible ways to use the strengths of the various classes in dungeon/underground situations, new prestige classes with obvious dungeon focus, new feets, skills, ways to use old skills, new uses for old equipment, new equipment and magic items.
On the DM side, discussion about dungeon design, ecology, trap usage/placement/rationalization... all that "why is the dungeon here in the first place", and "why is it the way it is" stuff you'd expect.
overall I'm very pleased with the money I put out on it. When I have time for a deeper read I'll know better just how pleased, but unless there are serious balance issues or broken mechanics, I'm expecting to remain happy with it.