I had an opportunity to try this out yesterday as I was on a reasonably quiet motorway and I must say that I was impressed - although it feels strange not being in control of the car. I am not sure how easy it would be to use on a busy motorway - perhaps one day I will build up enough confidence to try it!
One minor issue that I noticed was that it I was in, say, the middle lane with CC set to 70mph and the vehicle in front was travelling at 60mph then as you would expect my car slowed down. When I then moved out to overtake the car did not speed up until it was fully in the outside lane with nothing in front - whereas if I was driving "manually" I would be accelerating as I pulled out to overtake.
On a busy motorway it still works great... but on the shortest distance setting the gap between you and the car in front of you is still big enough to have other cars squize in... so you wil get some more abrubt breaking. But it wil still function fine.
When overtaking, just apply some throttle and as soon as you ar fully in the next lane release the throttle and you wil cruise on at the set speed.