When I was in Seattle, I tried out the first mission in Halo 3 in Heroic mode (third up of four possible skill levels; those being Easy, Normal, Heroic, and Legendary). It was hard. Actually, it was really hard.
Since getting home, however, I've decided to turn it up a notch, so to speak and go for it on Legendary.
It's stupid hard.
It's unclear to me that I'll ever be able to finish it in this mode, but I'll give it a shot. I've already completed the first mission on Legendary and now my son and I are split-screening on mission 2. (And in a continuing issue with Halo 3's graphics, the split-screen doesn't even take up the whole screen. So not only does each player only get a portion of the whole viewable screen area, but now we both get to squint into these even smaller than necessary views-within-a-view. Doy.)
We have two Xbox 360s in the office (two Halo 3 game discs) so we'll try co-op over the network later as well. Hopefully it looks and works better than with two people on a single Xbox.
UPDATE: Mark and I were able to finish Mission 2 pretty handily on co-op. That's a good sign, though I'm still dreading Mission 8, which was where things got really sticky, even on Normal mode. More tomorrow...