I agree with Posner that Buckley was not a major originator of ideas. He was, however, an absolutely superb public intellectual who had the courage to be a conservative when that was still highly unpopular, especially in the New York City circles that Buckley inhabited. He persuaded many young college students that a conservative stance is a respectable position intellectually, and to this end founded the political youth movement Young Americans for Freedom. He influenced Barry Goldwater and Ronald Reagan, although he was a much less important influence on their thinking, or on that of Margaret Thatcher and other conservative political leaders, than more creative thinkers like Milton Friedman and Friedrich Hayek.
I did not know William Buckley personally, but I admired his multi-talented contributions. He was a media entrepreneur, as seen from the influence of The National Review, a magazine that he founded when conservative magazines were not popular, and supported financially for many years. He wrote a widely read syndicated column On the Right, and hosted for many years Firing Line, a weekly television program that debated public policy issues. I have only occasionally read National Review or his columns, or watched his television programs, but I usually enjoyed and admired them when I did. When young he had repugnant views on several issues, like racial matters and McCarthyism, but he had the intellectual honesty to eventually repudiate most of these opinions. Over time he became the favorite conservative of liberals for his wit, use of language, and urbane debating skills...
Buckley was not interested in the technical discussions of economists, and the nitty-gritty economic issues they analyzed, but he was drawn to a generally free market position because he was attracted by the broader virtues of free markets and capitalism in encouraging economic, political, and civil freedoms. He did not care as much as economists do that, for example, agricultural price supports make for inefficient food production, or that tariffs on imports raised the cost to consumers of various goods. On the other hand, he did care very much that these and other interferences tend to stifle various freedoms. In this way he was, I believe, greatly influenced by the broader perspective of the effects of an economic system on individual responsibility taken by classical economists and the leading free market economists of his time.
See this interesting discussion with Buckley from Book TV