I'll know more soon but I seem to recall that since ACC is radar based and not laser based, I don't think dirt affects things as much...

 

I have comfort access, night vision and active cruise control. Of the three, the only one I would probably order today if I had to do it over again is the comfort access. I like not having to take the key out of your pocket to unlock the doors, start the car and lock it again. The night vision is cool and passengers are impressed, but it's not worth the money. You have to take your eyes off the road to keep checking it on the I-Drive screen and that doesn't make much sense if you're driving on a dark country road (which is where you would use it). I find I really never use it anymore. The active cruise control works great on the open road with a minimum amount of traffic. When there's any significant traffic with cars changing lanes in front of you, the sensor picks up the car and you are continually slowing down and speeding up. It would be great if there was a way to use the "active" mode when you needed it and to bypass it and have it act as normal cruise control when you don't. Unfortunately there is no way to bypass the "active"mode - you can just adjust the distance setting.

 

Having a "bypass" for the "active" part of Active Cruise Control has been on our wish list for some time now. The main reason I didn't get it on my current car was due to the exact issue you describe -- people cutting in front of you will cause the car to slow.

I ordered it on the new car thinking that the stop & go feature might be worthwhile in traffic so we'll see how that goes in about a week.

As far as bypassing the ACC on LCI models, the following post describes a technique that might be worth a try on an LCI car. Iceman tried it on his pre-LCI car and it didn't work but I don't know if it's been tested on a newer model. Give it a try and report your findings back in that thread...
http://forums.e60.net/index.php?showtopic=41442