Impulsive and 2005vzute have pretty much covered it, but I'll add my 2c in.
The button is used to turn the CC unit on or off, and activates the "CRUISE" icon on the dash. If the cruise control is actively maintaining the vehicle's speed, pressing the buttone once will deacitvate the cruise control, but not turn the unit off.
Flicking the knob down at a certain speed will set the CC to the current speed (over 40kmph), and cause the "ACTIVE" icon to appear on the dash. If the CC is already set and active, flicking the knob down will decrease the set speed in 1kmph increments; holding it down will decrease set speed until the knob is released. It doesn't apply brakes!
Flicking the knob up will resume the last set speed after you've deactivated CC, eg. for braking behind a truck. Flicking it upwards if the CC is set will increase the set speed by 1kmph each time; holding it increases the speed until the knob is realeased.
Pressing the brake (and clutch if manual) will deactivate the CC, but not turn the unit off.
gregiem, I have never noticed that before :S anyone else vouch for that? I don't recall mine requiring the brakes depressed once before it works...