I've played against daemons a few times since they came out. I have to admit I am really impressed with the book. The units all do interesting things, you can field a boat load of models and you can make use of that microsoft exel course your job made you take!
I think people are still sorting out how to play daemons effectively; but, so far from what I have seen it has been mad rushes straight into the teeth of my guns. I think that people are over estimating how survivable 20 T3 5++ models are.
I've really only seen two list iterations so far. The first is the pink horror list. 2-4 units of 20 horrors with heralds, a few chariots and some counter-assault unit. Pink horrors are a pretty damned good unit. the XD6 (X being what ever number between 2 and 4) shots really add up. teamed with prescience and the ignores cover rule you really have a wall of bullets. That is until you actually face something really scary. Like a librarian. A single librarian simply close enough to the targeted unit can nullify the power entirely on a 5 up. teamed up with a Codex marine set of power like null zone and gate and you will remove models by the handful! Pink Horrors need supporting units. Something that can add to their fire reliably and diversify their damage out put.
The next list is the Slaanesh Blitz list. This list is a boat load of daemoettes, seekers andKhorne hounds. They fly up the board in a hopes to catch and kill you before you even leave your deployment zone. I've only played this list a couple times but both times I was unimpressed. It could be the fact I am playing power armoured armies; but, whenever i play this list they get to my lines and are driven back with bolter fire.
I think massive numbers is the way for daemons to go, but i think you need to diversify you're lists. Slanneesh daemons with Tzeentch support gives you a lot of flex-ability. High rates of fire and movement. Add in some Chaos marine allies and you have some durable units to throw into the mix.
What do you think?