I used to play Halo 3 all the time. By far the funnest game I've ever played online. Funner than MW2. I like Halo better simply because you don't die as easily, forcing you to use more strategy in killing a person, rather than MW2 shows. Grenades are more useful. Killing people takes more strategy. In MW2, half the kills people get are luck. Whoever shoots or finds the other person first usually gets the kill. I do however, like how MW2 has a killstreak system where you can get rewards, as to where Halo doesn't. Two different styles and gameplays, I preferrably like Halo's more.
I liked ODST but it was way too short to buy. The night vision is very useful during the parts playing as the rookie. The firefight is good but you can get almost exactly the same thing in Gears of War 2's Horde mode. Better for renting than buying.
Me and my friends were playing Halo 3 last year and someone threw a plasma grenade. It was shot with a gun, then hit the lift and attached to the person that threw it. lol it was so funny