Downloaded the Halo 3 multiplayer demo. Played a game. 2 players got over 20 kills each, I got 1. I had forgotten how much I hate multiplayer FPS, running around and shooting at everything. It requires a different mindset than the single player game, and different skillset and tactics. I had hoped to be able to walk around and try out the new equipment, take my time. But, the cold map I played on is very small, with a lot of nooks and crannies. I wanted to be able to at least get an idea of weapon strengths and capabilities, but most of the time I didn't get to fire at anyone before I was shot in the back.
Gears of War multiplayer feels the most like single player of any shooter I've played, and I feel that my skillset translated over well. I still didn't enjoy it that much, and I only did it as a friend was online and asked me to join him.
Played a bunch of Call of Duty 2 multiplayer, again with a friend. I enjoyed this more than Halo, as the maps were a bunch bigger, and the weapons are scaled down. Still hated snipers and campers with a bead on the spawn area killing you the second you re-spawned and took a step. I also enjoyed the team aspect, rather than the free for all in the H3 game I played.
I'll still buy Halo 3, and play through the single player stuff. I may even buy Shadowrun, just for the single player training and Me vs Bots play. I really enjoyed the single player part of Rainbow Six Vegas.
Multiplayer Halo 3 may be fine and fun for some, but multiplayer isn't enjoyable for me. I DO love Co-Op.