This topic has been hit a lot lately on various blogs, so I'm not going to argue on either side in any depth. Let's just say that I'm still building my army and that I'm still intending on playing this summer and doing well. Read that as you will. If you build your list to beat Daemons, that sucks for me and mine, but good luck and godspeed to you in the rest of any tournament.
I'd like to discuss what I think Daemon players will have to do in order to hang with the GK. It's going to be a tough match-up because whether or not they go henchmen-heavy or GK-heavy they have rules specifically designed to bone Daemons. I think there may be a few changes to the fundamentals of how Daemons play that must be considered. You can't change your overall army list too much or you are falling into the same trap as a GK player trying to focus on Daemons. Anyways, here are my thoughts.
1. To go first or second: I can't say that this is universal, but typically I want to go second with my Daemons. I get to see how you set up and avoid anything scary, and I avoid a full round of shooting. However, depending on the build, I think it may be mandatory for Daemons to go first at times. Strike Squads and Coteaz can shut off the majority of the board if given a chance. There could literally be like a 4-6 safe spots to drop the entirety of your army and that's if it doesn't scatter. That is crippling.
2. Pavane is your friend: I like Pavane, so maybe I'm just biased. It's just fun to move your opponent's army. However, I think it will be more useful against GK than against other armies, especially with stupid Halberds. (I run a fiend-heavy list, and I6 sucks.) It is imperative that you control the flow of the board and choose when and where you assault/get assaulted. Moving a unit up to 6" can prevent a charge, allow you to multi-charge, or at least force your opponent to roll difficult. It is utterly imperative that Daemons get the charge against GK. (Stupid Psyk-Outs) Yes, I6 will still go first, but most likely your opponent will not have the whole unit loaded out that way, so at least you'll get a few guys to attack, and a few Fiends can be a beautiful thing.
3. Greater Daemons: I know that not everybody runs Greaters. Heralds of Tzeentch are great, but running several of them doesn't really fit with how I like to play. I've playtested both a KoS and BT. I'm still not sure where I'm going to land. KoS is awesome with his I10 and grenades. (I never realized how important grenades were until I didn't have them.) I10 will allow you to kill 3-4 before they ever get to swing, as long as you don't get charged. (See #2) The BT is great because he can insta-kill Paladins and WS10 is huge. "Yes, you do get to swing first with your Halberds, but you need 5s to hit and and 5s or 6s to wound depending on Hammerhand." Also, Iron Hide makes him more survivable against shooting than the KoS. Lastly, a 2+ Inv versus force weapons is awesome.
Well, those are my thoughts for tonight. I have to get back to putting Horrors together.