Way back when I first started online gaming about 9 years ago with CS 1.5 and Elite Force, it was atrocious. Basically one out of every three servers you played on would have several speed hackers and such.
As a PC Gamer pretty much exclusively (I have a 360 and a Wii, but both are gathering dust somewhere not even connected to a TV) I don't think I've really encountered a major problem with it at all recently, though.
Bad Company 2 had a few aimbot exploits back when it was first released, but punkbuster isn't actually as rubbish as it used to be, and you normally get a pretty good game on it now.
In fact, I think the only game I still play which has a cheater problem is GTAIV, and nobody plays that competetively anyway, so why complain?
Actually, half-life had a hacker problem before. When they were making HL2, somebody hacked it, and they ended up removing the Lost Coast section.By hacker he means cheater, not an actual hacker. The term has been used instead of "cheater" for as long as I've played video games. No idea why.