People are so funny... they hate Halo for being popular. Halo is by far the most fun game to play with friends. 90% of the people who bash it have never even played it, or just played it once or twice.
Halo is not over rated at all. Games that aren't great don't become as huge as Halo. Maybe they can become big, but you can't reach Halo level without an insane amount of quality. It changed the face of FPS's forever.