You guys forgot to mention that he needed the wireless receiver if he's using wireless, it's about $15-20. One annoyance is, if the controller is registered to your xbox also, you have to turn your xbox off when you turn on your controller, but there's no problems with the controller being registered to both.
Also there's no way to turn off the controller, just let the controller turn off by itself. No big deal, turns off after 5-10 minutes.
You can also use the PS3 controller, but you have to use the usb dongle and some software. Get the software off the net.
L4D isn't that bad to use with controller, because you don't have to be so accurate since it's not a 1 shot your dead type FPS. If your accuracy is bad, you'll hit your teammates often, and they'll be pissed off at you!!!! If you have an xbox and and a gaming PC, get the PC version. Playing with the controller, you'd think you were on the xbox, but with better graphics and you can avoid most of the annoying kids on Xbox Live. Plus the mods and free DLC is a plus. And PC games are generally cheaper.
I prefer kb/m, but I'm pretty good with the controller also. I'll hook up the controller when I want to lay back and be in a more relaxed position and play. Use a mic, otherwise it kind of defeats the purpose of not using the kb. As far as accuracy, headshots, not hitting teammates...etc is concerned, I'm usually 1st or 2nd in the ending stats in the majority of the categories. Then I'll say it at the end that I was using the controller. Some people can't believe me, others will say it's not a big deal.
I'm better with kb/m, but it's more relaxing with the controller. If playing with the controller is better in both comfort and accuracy for you, then just do it. Who cares what people say.