First impressions are critical and Cameron ensured these were good. He may have had an easy ride from a media keen to see a more effective Opposition, but he has achieved a lot more than Tony Blair did in his first 100 days as Labour leader.
Cameron has been upfront with his party about the changes it needs to make to become electable again. He's been a little unlucky in that - except for a minor spat with Lord Tebbit and letters to The Daily Telegraph - he has not provoked the row with the old guard that Blair managed: it was this, and not the ditching of Labour's historic 'Clause 4' commitment to nationalisation (which no one had any intention of implementing), that helped establish Blair as a 'New Labour' leader.
Blair didn't change any Labour policies in his first 100 days. Compare that with Cameron, who has just about junked the lot.
The Tory leader has been even more active than Blair on the all-important image front too. We have seen Cameron cycling to work, applying for a wind generator on his house and showing off his wife's tattoo. All Blair did was to appoint Alastair Campbell with all the problems that would eventually result. Cameron and his team have been spinning like tops, but it has been done without high-profile spin doctors.
And Cameron has played a blinder over the Prime Minister's difficulties with his Education Bill. For the first time in years, commentators are now saying that the Tories could win an election.
However, the Tory leader's big test has yet to come. Next week he will face Gordon Brown across the dispatch box after the Budget statement.
His strategy of allowing George Osborne to make personal attacks on the Chancellor was probably a mistake. It will make Brown doubly determined to do well. After savaging the shadow chancellor the first time he crossed him, Brown could give Cameron the same treatment. He just doesn't like public schoolboys.
So long as Blair sticks around, Cameron does not need to criticise him, just agree with him. All his energy can be devoted to attacking the man he will face at the next election.
Brown cannot fight back in the way he wants until Blair quits, and there is little likelihood of that in anything like another 100 days. This gives Cameron time to establish himself as the most effective Tory leader since Margaret Thatcher.