People who try to down halo have yet to see the competitive side of it. I really couldn't give a flying crap about its story line or campaign. Its funny how the game is rated based off of that by 90% of people. Kids freak and obsess over the game for its crappy story line, and care little about multiplayer.
Halo on xbox live is where it shines. People who try halo, who come from PC games really havn't touched on this. (above posts talking about how 'good' splitscreen is.)
People who have played xbox live and dislike it who come from PC games have been blinded by the thousands of kids who play live to "pwnz0rz d3m n33b5".
Halo is one of the very few console games that actually has complex skill depth to it.
People who constantly play other games such as gears and ramble about halo's "suckiness", are utterly terrible at the game. A good halo player can go into a game like gears, and after a week can be as good as everyone else. Where as someone from a shallow skill game like gears tries to come to halo, and will get mopped 50 to 0 in a 1v1.
Those people make up the excuses that the game sucks and is over-rated for reasons like "its so unrealistic how he jumps 10 feet in the air!".
Halo has a skill depth to the point where there are professional MLG players who make millions playing HALO. (ie Walshy)
Many people hate it because they are unable to handle aiming, shooting, and timing grenades, jumps, shots, melees and movement at the same time; quite frankly, because they get killed by those who can. The only other game that really can be compared to halo as far as skill goes with fps would be shadowrun. Shadowrun is one of the other few that has the jump aspect, prolonged rewareds for better aim (as in it matters if you miss 1 headshot in a fight), and where the more skilled player can win a fight starting shooting later or starting with lower health.
Halo is overrated for the wrong reasons: its campaign and storyline. Its underrated for its multiplayer gameplay, skill depth, and competitive side.
I suggest to those who dislike halo because of the boring campaign and standard xbox live kiddies, to look into the competitive side and mlg part of halo.
EDIT: Also, I frequently play both console and pc games. I'm only comparing halo to other console games.