After Reach was released, obviously the population of Halo 3 plummeted. However I've been playing Halo 3 a lot more lately, and at peak times of the day, the numbers seem to be going up gradually? At weekends, there's definitely a healthy number of people on.
It just makes me happy to still see people playing it. I've not come across cheaters, just good-old, healthy Halo 3 action. Can't imagine it getting old.