When I first got my 360, I only had money for one title (Mass Effect), so I borrowed Halo 3 from a buddy. I hadn't played the other two before, so this was my first crack at the series.
I was utterly disappointed.
I couldn't believe how crappy the graphics were. There were these big open environments, and you couldn't always tell where the enemies were because if they were too far off, everything got all pixelated and weird looking. There was very little evidence of any anti-aliasing as well. In general, I was just very disenchanted with the game, and didn't really have much fun playing it because I was always straining to see what was going on. I haven't touched the game since. I would consider Halo Reach if it looks a little more polished and actually has fun gameplay.