I've used both wireless and wired allot! Especially in FPS games. Here's my take on it, the best keyboards out there are wired(mostly with macro's, mechanical, backlit, palmrest, etc etc).
Wireless keyboards on the other hand doesn't come in a huge variety, so you'll be stuck with some office keyboard.
Another point is that wireless keyboard really does have a small lag, especially if you have other wireless peripherals near you. I've been playing BF3 online with a wireless keyboard for some time, I did good, but once in every 2 hours there would be a small lag(keyboard would stop working for a 2-3 seconds). But I can say that the games are playable.
Just make sure you put the receiver somewhere close/clear near the keyboard.