Personally, I don't have too much of problem in principle with using a mobile for a conversation when driving, but then I drive an automatic and I reckon that 95% of the time I can control my car perfectly well with one hand. I doubt if the distraction level is significantly greater than having your passengers arguing amongst themselves and with you. A crap navigator can cause even more distraction.
BUT, having said that, I obey the law - I use the built in car phone if absolutely essential, and I have a simple hands-free amplifier for incoming calls on my regular mobile. Apart from motorway or dual carriageway cruising, I can't understand how anyone can safely manage a manual car while using a phone. Texting is just plain stupid.
It is often said that speed kills. That's true, because if you are involved in an accident, the higher the impact velocity, the more severely you are likely to be injured and ultimately killed - its simple physics. But it isn't necessarily speed that triggers accidents. In my not very humble opinion, stupidity is what causes accidents.