In the context of the fact that I strongly dislike 6th Ed, I think this is a decent Codex.
I've just bought it and skimmed it (as such, if I got something wrong, my bad).
Things that suck
Drop in Letter & Bearer T - given that Marines find them hard-going in the fluff the previous T values seemed appropriate, T3 5+ is pony in Fantasy where people aren't shooting you with Boltguns
The Warpstorm table
Rewards being random (I know there's a default, this mechanism is &^%* for Spells, &^%* for Psychic Powers and &^%* for rewards, get over it already
GW)
The Masque
"true" Psychic Powers - I don't object to this in principle, but they should have changed Psychic Hoods et al to cost a Warp Charge to use so you can overwhelm the enemy instead of some dude sitting there with a 24" exclusion zone, but Speese Mahreens
Chariots - though that is a 6th Ed/main rules issue, Daemons just suffer from having several of them
No fliers. Seriously? Seriously?? SERIOUSLY??!! Dammit
GW with your £££ chasing cynicism you couldn't crowbar the Helldrake with unbroken rules in here?!
Should have made the named Heralds W3 to give a "captain-equivalent"
HQ, it jumps from W2 glass cannons to Daemon Princes with Karanak as the only real bridge (you can mount Khorne or Nurgle Heralds to get more W) - Skulltaker particularly suffers as Mr Uber-challenge is fairly pussy
The Double 1/Double 6 results on Daemonic Instability
Want to hurt flyers? Soulgrinder or Vector-striking with
BT/
DP/
LoC only
Throne of Khorne.....just what?
Lord of Change only gets 2 rulebook disciplines....Speese Mahreens
Goddamn where's my Slaanesh GD special character?!
Warpflame
Formatting on Melee Weapons - do they need "range", can we drop this column so the "Type" field isn't 2 miles long?
Taking a GD pushes
DPs of the same God to Heavy Support...WTF? So mono-god gets humped on trying to include Skyfire? This...I....just?!
The Hellflayer (especially in comparison to a normal Chariot) - I just don't get it, why does the rolling pin of doom not have a material difference?
Can haz more Daemon Engine type stuff plox?
Things I like
Daemonic Alignment
Daemonic Instability as a means to not have an entirely unbreakable army (subject to complaint above)
Greater Daemons now have appropriate stats IMO
Deep Strike entirely optional
Blue Horrors
Beasts of Nurgle counter-charge
Fiends Soporific Musk
Loci
Icons and maybe Instruments now worth taking
Heavy & Fast sections worth a damn at last
I know the first list is longer, but I actually overall like this Codex, it's the first product outside of
FW in a long, long, long time that I've actually picked up and felt reasonably enthusiastic about - shame about 6th and the Warp Storm table.
Not about to go throwing any money about but got enough old metals and stuff I can probably throw a 1k army together.