For those interested, the factory radar cruise can apply the brakes HARD if it feels the need (it has activated the ABS in mine before), and can slow the car down to around 5km/h or so. It might be able to stop completely, though I haven't been game enough to let it try.
If while driving you catch another car and the speed difference is too great, it will beep to indicate that another car is within the range of the radar, and either roll off the throttle or brake gently to maintain the safety gap distance you have set. If the speed difference isn't too high, it will just match speeds with the car in front without a beep.
The safety gaps are "far, very far, and stupid far"
It will only apply the brakes to maintain the set cruise speed if you are following another car, so on a steep downhill it will basically free roll. At 120km/h the cruise beeps and disengages.
Occasionally it will have a fit at some odd-angled road signs, when driving into morning/evening sunstrike, and it will sometimes 'see' oncoming traffic as being in your lane. Most of the time these events just produce a beep, as the object passes by and the radar clears. In some cases it will stamp on the brakes, though again only for a brief moment (though this is annoying for people following)
Fortunately in all cases it beeps which gives you time to react. Once you know what to look out for, you can predict it's behavior.
The brakes are activated by an actuator which actually pushes the brake pedal, it's creepy to feel it moving.
The cruise won't engage below 45km/h, or if the gearbox is in manual mode or snow mode, or if the windscreen wipers are on normal or fast. It also won't engage if the radar unit is obscured/disconnected.
It will disengage if the front wipers are turned on for more than 10 seconds or so (though it is happy to operate if the wipers are only on the intermittent setting). It will also disengage if the rear wheels slip, if the sensor becomes too dirty (following cattle trucks etc), if you corner too hard, or if the car aquaplanes. The cornering one I discovered playing on a track - you'd have to be *really* pushing on the road.
It must have a G or steering angle sensor somewhere also, as it will often allow the speed to drop while cornering, and then accelerate back to the set speed once the wheel is straight again. I guess this avoids ending up with a heap more power being applied mid-corner.
It's not without it's problems, but it's a damn clever system. Just a shame it can't be set over 110km/h - my speedo reads 10km/h low at 100km/h.