I agree, the demo is really fun. The Cut Scene graphics are ten times better then Halo 3's...as are the in-game graphics. Much more realistic.
Also, I tried out Skirmish with both the Covenant and the humans, and I prefer the Covenant a lot more. I actually built a Scarab...that thing is ridiculously powerful, as well as ridiculously fun to use
Tried to attack the human base with two Wraiths and a few squads of Jackals, only managed to take it down to 75% health. Sent in the Scarab, base was reduced to ash in seconds.