You have overview of the story, with monsters/items in one booklet.
In the other you have teh adventure "path", and set encounters on the path.
This is the important part: The format is very effective for the "string of pre-designed encounters" style of running a campaign. It is, however, extremely ineffective for other styles of running a campaign (emphasizing, as it does, a lack of interaction within a complex).
A few other thoughts:
(1) While I'm a big fan of not splitting up a single chunk of information across more than one page, the actual strictures of the format seem to frequently end up being the tail wagging the dog. Encounters that wouldn't necessarily fill two pages are either eliminated or bloated up until they do fit on two pages. Longer encounters are occasionally allowed to spill over onto additional pages, but it also seems like those get discriminated against in the design process.
(2) The sub-maps for each encounter don't seem to have much utility for me. I've got the map of the complete complex for reference. And if I'm playing with a battlemat, then I can just look at the map I've drawn or otherwise laid out on the mat. The only utility the map has is to show me the starting positions of the monsters, and (a) I don't really need that and (b) if I did, it could be easily included on the map of the full complex.
There is a tiny subset of encounters where additional details not possible on the large map are useful. But these are the exception, not the rule.
(3) The early uses of the Delve format where information about a single location was split up into two different locations in the same book was an incredibly bad idea. Fortunately, they've abandoned that misguided approach with more recent modules.
EDIT: Okay, I thought they'd moved away from it. But others are still reporting its existence. Ah, well. I thought they'd learned.
So, all-in-all, I'd call the format to be a bit of a mixed bag.
Honestly, I wish they'd go even further. Why have maps with only the initial positions of bad guys? Add in all the things you need to know about the room ON the map. If a fight in room X brings reinforcements from room Y in Z rounds, put that on the map. If there are lighting issues, or any sort of terrain issues, put that on the map.
I should be able to look at the map and get a very good overview of the entire adventure without even opening the module. The location of every enemy, every trap, every terrain feature, and probably more information, should be on the DM's map.
This I agree with completely.
Features I'd like to see in a module:
(1) A keyed listing of monsters by location, allowing for (a) easy modification and (b) running encounters dynamically.
(2) Within each encounter area, I want each major element of the area bullet-pointed. (This way I can quickly glance at the bullet-point titles and know what's in the area. And, when I need specific information on any given element in the area, I can quickly find it.)
(3) Properly constructed boxed text that describes an encounter area without assuming that the characters are taking a particular action. (Even if you don't read the boxed text, this neatly packages the information that PCs should know "at a glance" so that you (a) don't forget anything and (b) don't tell them too much.)
(4) Every single illustration in the module should be something I can show the players and say, "You see this." (Pretty pictures of fictional PCs fighting the monsters or exploring the dungeon are pointless. Why give me illustrations without utility when you could be giving me illustrations with utility?)
(5) Try to favor a layout where chunks of information that will be used at the same time are not split across multiple pages. If it is necessary, try to have the information split across facing pages. (This will not always be possible, but an effort should be made.)
(6) Everything I need for a particular encounter/area should be in the description of the encounter/area. If not, I want a very specific page number so that I can find the information quickly.
(7) If your cartography is gorgeous, make it possible for me to share it with my players. In the digital age there is simply no excuse for not making the maps available for download in size and resolution that makes printing out battlemaps simple and straight-forward.