Its not actually as bad as they make it out to be.
The way it works is like this. If player A has a line of sight on player B, and fires (with a hitscan weapon only [those are the "instant hit" weapons, btw]) then when the signal to fire actually gets to the server, it counts as a hit. Basically it takes the server's ability to register a hit, and puts it in the hands of the client.
Alot of people think this is cheap because "you can just unplug your ethernet card or telephone line, fire, and plug it back in to shoot around corners", but this is not so. First of all, if you are killed before the shot is registered, you die, and the spawning cancels the shot. Second off, you need to has a clear LOS to the target you shot at the time the signal goes through, meaning if they duck behind a wall or something while you're lagging, you do not get the kill. All it really does is say "okay, you were actually fast enough to hit this guy, but your modem wasn't, so what I'll do is since he moved n paces this way, I'll turn you n degrees this way, so that your hit registers".
Its perfectly fair, and legit, and it makes perfect sense. Its just a way for hitscan weapons to hit as if they were fired normally, but if your lagging pretty bad because higher band-width players are taking up the server's juice, and you start dropping a bit, well... it makes it a little bit more fair. This way not only does the guy who shells out $35 bucks a month for cable get a decent score, but so do you, because you were always fast enough, its just that damn telephone line that wasn't.
I said that already though. But it is a really fair, decent way to play. Doesn't work with projectiles for obvious reasons, and it of course doesn't work if you break LOS. Anybody who tells you its unfair is full of shit, and they're only saying that because their mad amounts of money every month for broad-band access are no longer making them look as good as it used to. Server Nazis.