I ended up playing my daemons again for a good Monday evening jaunt. Of the game I have played, 3 have been against grey knights and of those 3, I have been able to pull out a victory. Maybe it is that I am playing daemons that do not fall prey as easily to the strengths of the grey knights, or it is certainly possible that I am out playing my opponents, but really I think it is a matter of perception. Although grey knights have some fun toys to use against daemons, they are still an ultra elite army, and each loss is a huge hit to them. Where as daemons are a pseudo horde force that relies more on expend units to strike at the heart of their opponents.
First, I think most people are expecting heavy close combat daemons. Specifically the ever popular fate-crusher, which is not even close to what I play. In fact I probably play one of the most shooting heavy daemon armies you can. I typically run 4 heralds, 6 units horrors, 3 units of flamers, and 3 princes with 3 units of screamers to deal with heavy armor. So in the case of grey knights, I have less concern around warp quake and such, and I can focus on real damage dealing and placement. Additionally, the copious amounts of ap3 and such going around does not hurt against typically marine heavy armies.
Secondly, mobility. Most of the items in my list are fairly quick. There are still walking units, but the heavy hitters, flamers/heralds/screamers are all very quick and able to bounce around the board and do their work. Mobility and firepower are some strong points to keep you on top of your opponent.
All in all, I am more surprised at the effectiveness that the daemons have had than anything else. They have taken several games that I should have lost, or at least taken them by a smaller margin of victory.