It seems like 360 Left 4 Dead heads were met with a bit of a letdown during the first week, and encountered a great deal of lag: some consumers have even reported that they passed on the game because of it. Well fear that invisible beast no more!
In a move that Valve calls a “massive server upgrade“, it seems like lag issues are down to a minimum. In the words of Valve: “now quit reading this; get out there and kill some zombies!”
Via Joystiq

Well, it seems like I made a good choice in waiting to get Left 4 Dead 2, since I was planning on getting the 360 version.
None of my friends have complained about any lag, but it’s too hear that Valve is on top of any problems that do exist.