I played the halo 3 beta for a few hours one night when my buddy brought it up to my house. I thought it was "alright", but thats it...nothing uber cool or anything. I was frankly expecting a lot more refined gameplay from halo2 , bu as it turns out it's almost exactly the same. The 'ol..."who ever shoots first wins games play". I don't mind that style of gameplay too much but i dunno if it's gonna be as big a hit to me as i "hoped" that it would be. 
As it stands right now, the game play is so similar too Halo2 i don't know how anyone who never played that game alot is goning to stand a chance online. Talk about frustrating gaming....unless you play (like we do) with people you know! 
Thankfully, there's a boat load of good shooters coming out....so there will be a lot too chose from. 
Now if only my god damn Playstation 3 would release a good game.....