I've never played Halo nor Halo 2. I've now played the first level ("Groundfall" or somesuch) of Halo 3. Gotta' tell you: my initial impression is that it's not as entrancing as Gears of War or Call of Duty. The cutscenes make absolutely no sense to a newbie, the level design was nothing special (oh, the trees shake when you move past them. Cute.), the combat tactics weren't clear, and the controls seemed sub-optimal (I read in Wired that you were supposed to always be switching between grenades, guns, and fists. But to use your fists, you have to take your thumb off the "look" control, so it's hard to melee).
People talk about multiplayer, but let me clue you in to something about multiplayer: it's offputting to normal people. Normal people don't enjoy undergoing a series of losses before being buoyed up by the matchmaking system. Perhaps subsequent to the 10th match everything is wonderful as you slot into the ranks of your peers. But I'll tell you what -- I've never played 10 matches online in any game, because my first half-dozen matches have always been unpleasant. In racing games, people intentionally crash through the first corner; in first person shooters, you get picked off ten times in a single match.