In college, I was stuck in a video game purchasing mode that was uncontrollable. I may have owned over 40 Playstation 2 games at one time. Over the past five years, I've learned to do a lot research into games I purchase. Re-playability and bang for your buck are the two biggest factors for me. I'm a big fan of XBox live achievements also.
The Left 4 Dead series can be one of my favorite games that offers a completely unique experience every time you play. Both games have five campaigns. Each campaign has four to five chapters. Both games have a DLC campaign that have two or three chapters. A third DLC campaign is scheduled for release October 5th that will feature the characters from the first game. It looks like it is available to download for each game with different features for both games.
Starting with the second game and jumping down the first one can be kinda rough. The second game features an additional weapon tier and melee weapons that really help out with big crowds of zombies. The first game only has single and dual pistols as your "backup" weapons.
The first tier weapons include a pump shotgun and uzi style full machine gun. The second tier has an auto shotgun, assault rifle, and semi-auto sniper rifle. The second game adds an additional version of each weapon to each tier that has a different ROF, magazine capacity, accuracy, or damage rating. The second tier shotguns are the tactical and combat shotgun. The tactical shot gun has a wider spread making it better for closer ranges than the combat shotgun.
The "Director" is the true villain of the game. The directer determines item, weapon and enemy placement. You'll encounter boss infected, called Tanks, in different locations every time you play a campaign. The same applies for all the weapons and item pickups. This means that playing on "normal" could end up being as difficult as "hard" depending on how the director is acting.
The series is great. It's even better when you play with a party online. Regardless of which one you purchase, expect a lot of play out of the game without getting bored. I'm really hoping a third game comes soon.