First, I really do like the cover art. It gives the early impression of the shadowy nature of the army, and gives the impression that they're very different from the other races. While most of the other codexes (especially imperium codicies) use bright colors and showcase heroic acts, this shows a night time raid of them slaughtering some helpless imperial guardsmen. It gives the image of the Necromongers from the 'Chronicles of Riddick.'
In the early sections, it gives a good telling of the dark eldar fluff, how they're different from their Craftworld cousins (Dark Eldar are not psykers, nor do they have psychic powers!), and some background about their city of Commorragh hidden in the webway. There's also some interesting stories about the rise of Asdrubael Vect as the Supreme Overlord of Commorragh.
Once you get to the actual rules, it's laid out similarly to other 5th ed codices (page with overall army rules, each unit with fluff and a few rules, wargear section, and then army list section in the back).
I do have to say that I really like that this codex is written much better than other recent codicies. The special rules and abilities are written in clearer language and often come with specific examples to illustrate them. It really goes a long way in clearing up many potential rules issues. I'm sure there will be some rules issues that arise. Nearly all of the them that I could see regarding many special character abilities and army rules are very well written, leaving little question as to what they mean.
There is a massive amount of wargear in this book, nearly all of it is completely new or has been changed since the last book. Even familiar items such as Dark Lances, Agonizers, and shadow fields have had points costs changed or details changed (Agonizers no longer glance vehicles on a 6). I highly recommend everyone that either goes to tournaments or plays against dark eldar players to get this codex, just so they can have an idea of what all of the wargear and stuff does.
As to how the army will do, there is alot of potential for a variety of things. The overall theme of the army is a glass hammer / glass cannon. The army can deal a heavy amount of damage, but is relatively fragile and has a low model count. There's 8 different unique characters, where I could see myself choosing between 5 or 6 of them deciding what I want to run. Every slot is packed in a similar way (especially the elites slot), but unlike other codexes, the layout of the army doesn't force you into taking any particular choice, it really is up to what you want to run.
What to do with the army? There's many possibilities.
There is only one bad unit I see out of this whole book, which unfortunately is still the mandrake. Despite the awesome looking models coming out, they seem to be the weakest unit in the book. They have some improved abilities (so they're not as bad as a chaos spawn), but overall everything else in the elites choice seems better.
One of my complaints of the codex is just the wargear being scattered similarly to other 5th ed codicies, but it's really a problem with this book as there is so much new and different wargear (not like imperium weapons, where the majority of it is standard from space marines to blood angels and space wolves). It makes it kinda time consuming to sort through all of the sections to figure out what each piece of wargear does.
As people are already dissecting this book and deciding upon tournament builds, I'd just suggest everybody just experiment and try new stuff out with this army. Don't rely on internet lists, as everyone is starting from scratch with this book. The power level of the codex seems similar to a blood angels level, I don't think it'll overtake Space Wolves or Imperial Guard as a game-breaking codex, but is good enough that good players should be able to win well with this.
As people have complained forever that GW doesn't support xenos races; this has answered that claim strongly. It is a well written book launched along with fantastic looking models. It appears to be pretty balanced and will provide a different style of game apart from the MEQ dominated games normally seen. If GW can replicate the quality of this release with their upcoming xenos codicies, then the game will just keep getting better and better.