This is of course for the PC.
Unlike the Amazon pre-order at the same price, steam offers an exclusive early access to the demo, exclusive in-game baseball bat item as a melee weapon.
This is true also for the 4-pack deal posted:http://www.hotukdeals.com/deal/481995/left-4-dead-2-4-pack-80-99-works-ou/
The downside is of course you must then install Steam on your PC, and you have to download the game instead of getting a disc. Which also brings about the fact that you can't re-sell the game as 2nd hand copy.
Another deal is the bundle pack for Left4Dead 1 and 2 together for just £44.98.
This is a bargain if you don't have the first one, which is a great game in its own right.