The game defiantly starts to pick up a lot in the second half. I bought it day one and when I first started I was worrying it may not live up to the hype when I first started to play then about 30%-40% is were it really picked up for me, I couldn't put it down. It is the best game I played in a very long time and its my pick for Game of the Year. The multiplayer is also insanely fun once you get into it. Keep playing the game is awesome!
Edit: Also I would suggest anyone that wants to play it to play it on Survivor. It is much funnier and adds to the survival ascpect to the game making the battle scenes really, really tense and fun.