Okay, I'm no princess, but my story is rather similiar to William's. My father cheated on my mother with his boss' secretary. He humiliated my mother to the point she tried to kill herself.
My parents split when I was 5, one year later my father and his 'lover' moved in together. It's been 21 years and nowadays I'm good friends with her - who was the woman responsible for the wreck of my parents marriage - my mother ENCOURAGED this friendship.
I lost my mother at the age of 16 and not a day goes by without me thinking about her, missing her. I supposed that William missed Diana terribly during his graduation ceremony. I know I did miss my mom while I was going through my own graduation ceremony.
Bottom line is, it is better to cultivate a good relationship. Camilla is Charles' wife now, William and Harry had enough time to get used to this situation before she became an official member of the Royal House. She's their step-mother and never will take the place of Diana in their hearts, but that doesn't mean that the boys can't show affection to her.
Living in bitterness is bad. It makes you feel bad, it provokes illnesses. Besides, it could create a terrible environment for Charles, because he would be faced with the terrible choice: my sons or my wife?
Please, as someone who's lost her mother and has her father married to the woman responsible for the ending of his wedding, I think I can say that Wills is doing the best for him as a person and for the institution that he's part of that is known as the British Royal Family.