First things first: assuming that someone at Rainy Day Games reads this, I did manage to get a copy of Dungeon Delve before March 3rd. So, ha ha and stuff.
Dungeon Delve is pretty straightforward: each delve is three encounters strung together, and you get one delve per character level (for a total of thirty).
The book opens up explaining the origins of the delve before moving on to various uses for the product: to fill in an encounter, to run a very short game, to give someone else a chance to run a short game, run it as a competitive game between the party and DM, expanding upon a delve, and a page of all the new monsters featured in the book. Well, not their blocks, but the pages you can find them on.
The delves themselves are likewise straightforward: you go from room to room, engaging in encounter after encounter until you are done. Each delve tells you which Dungone Tiles set you need to build it, but if you dont have any you can alwasy draw the map as usual. They're all thematic in the sense that one has lots of fire monsters, and another one has lots of orcs. If nothing else are a good excuse to use your Dungeon Tiles and various monsters that you havent been able to use before (especially if you havent gotten into paragon or epic tier).
There werent any particularly fantastic locations for any of the fights. They're all self-contained dungeon setups, regardless as to what you are taking on. I would have liked to see planar locations or something more dynamic used in the paragon and epic delves.
The real question is why you would get this book. Well, a delve is really just a grand total of three encounters. Thanks to the simplicity of encounter building, I think its safe to say that anyone that can read and perform basic addition can build an encounter of any level. All the monsters synergize well in a level group, so its hard to not make an encounter without lots of similar monsters. Want to actually play an epic level character? Have your players make one and then build your own sequence of encounters for level 30 characters.
I suppose that this book is ideal for newer players, or a player that has never run before (in any edition). The designer sidebars might be useful for some, and more monsters are always good. Casual groups will enjoy it becuase its well suited for short, casual play, while hardcore groups will like it just because you can run them as player vs. DM events. What it comes down to is whether the money is worth saving the time it would take you to do it yourself.