Yesterday I played a 2000 pt game, my first using the new book.
I had a guardian host, seer council, wraith knight and hemlock.
As far as the Craftworld Warhost goes Matchless Agility is amazing, even more so when paired with something like The Quickening, because a 9" run guaranteed allows you to re-position in ways your opponents will have a hard time predicting. That said, a lot of players will dislike the tax of guardian units it takes to unlock the ability for normal games. This formation is truly designed for apocalypse games, most likely ones that are well into the 5000 pt + range, at which time it will actually simplify things like Battle Focus because you don't need to spend time rolling for each unit each turn... they just move.
In my game I used the defender host so I learned that preferred enemy on the Vypers and war walkers came in massively handy, converting some abysmal rolls to deadly fusillades (two times I rolled 4 1's in my shooting and converted 3 of the 4 to hits). In addition the free heavy weapon (they should have made it free per 10 guardians in the unit to encourage larger squads) will probably often be missile launchers, not only because it's the most expensive weapon, but because the anti-air missiles are included, which gives the Eldar a lot of casual access to anti air.
The seer council is bonkers. I used a full squad with Eldrad and they pretty much had their way with the board. Runes of fate is a very powerful tree now. That said, fortune being random is still a major detriment for regular use of the foot squad, because without it or invisibility the unit would have crumbled. Also sanctuary was massive. Without the sanctuary/fortune combo I would have lost 8 out of 10 warlocks in 2 turns, so they are very fragile, even if they were the cost of a Titan. For competitive players I don't expect you'll see the conclave outside of small jetbike squads and even then it will have a lower mastery level and provide fewer warp charge. Fortune + Protect will still be powerful, but far less likely to occur than previously given you're not getting a roll per warlock on runes of battle anymore.
The Wraithknight wasn't particularly impressive. Neither were any of the D weapons. Much ado about nothing. The only real difference between the old gun and the new was when I 6'd out a chaos lord so that he didn't get a 4++ that in the prior book he would have received against an instant death hit. The Hemlock's best effect was attained through psychic shriek rather than the scythes, and I think once players get used to the altered rules they'll find they're going crazy over fairly expensive units that are very good at specific jobs. The only time we'll probably see them be truly outrageous is in apocalypse where a unit of 10 can be paired up with an Archon and deep strike without scatter and quite likely wipe out any titan in the game in a single volley. I doubt there's an appropriate counter outside of Interceptor Tau, but you are unlikely to see that in anything outside of Apocalypse, and it probably won't be the only insane unit on the field.
I think the Eldar will be maligned for extreme lists like mine (at max strength it's 24 warp charge prior to the bonuses) in competitive play, but there will be a large number of players that ultimately will build flexible lists that incorporate a single formation to support a CAD.
In the long run I think opponents will adjust, but it will take time. Until then there will be much complaining and gnashing of teeth. Death Star builds in particular seem to be hard countered by Space elves, so expect many more complaints from players that depend on those lists than from MSU style lists. In my game my opponent made some early miscues, had a moment of bad luck with a failed charge (needed 5 or higher and rolled a 3 and a 1), yet because we were playing maelstrom at the point he surrendered in turn 4 the score was 5-3 in my favor. It was still close, despite most of his army being gunned down. If you're facing this codex do NOT try to take it on in waves. You will be eaten alive.
One thing is clear, the tournament scene is going to have a lot of soul searching in how it wants to handle 40k as a whole. In general the playstyle of the Eternal War missions versus the Maelstrom missions is totally different, and it's clear the designers are not setting balance based on Eternal War missions. If those are the foundation of your game good luck. Some armies have ridiculous bonuses in that play style. Maelstrom actually evens things out quite a bit making the game a mixture of army design, skill, luck, and adaptability over the course of an entire game.
I look forward to testing more builds and seeing what other combos this book has up its sleeve.