Hi Lexus
Firstly I assume your IS250 is Sport Luxury model, or else don't try the radar cruise control because it is not going to work in other models.
Radar cruise control is very useful, you can pretty much take your feet away from the pedals.
It is depended on how sensitive you set your radar regarding to your car cutting in issue. Your car will be set in the most sensitive radar default range after activated the cruise control, your car will be miles away (At least 15 car spaces distance @ 115km/h) from the car in front of you. Your car will slow down to maintain the same default distance with the car which cut in to you. However you can adjust the sensitivity of the radar.
Normally the little screen on the dash shows a little car and a big car with 3 beams when you activated the radar cruise control, press the right bottom button on your steering wheel to adjust the sensitivity of the radar while cruising. Then you will see one beam between the 2 cars is gone by pressing the button once, then again one more beam will be gone by pressing one more time. One more time will set it back to 3 beams. The less beam will give you less sensitivity of the radar which means the distance between the car in front of you is going to be closer but it is still a safety distance even you set it in 1 beam mode (about 5 to 6 car spaces @ 115km/h).
The only down side of setting it at 1 beam mode that you are still keeping a good gap for people to cut in but not as much as 3 beams mode. The car will slow down aggressively and making loud beeping sound if there is a car cut right in front of you in a 1 beam mode. Something you have to get used to it because it will happen!
One funny thing about the radar cruise control is sometimes I just let the car following the car in front of me and don't even realize I am following a slow car and doing 90km/h on the freeway!
