I met him only once, briefly, and it was this past summer. He was clearly diminished, but still remarkably sharp for a man in his 90s.
Buchanan's influence will be felt by those who study constitutional economics, law and economics, and theories of logrolling. My sense is that outside of those areas his (direct) influence has slipped in recent decades, although that impression may be incorrect. In any case, he helped resurrect political economy as a field of study distinct from welfare economics. I do a very different sort of political economy from him, and yet I am in his debt. Early neoliberal institutionalists in IPE built some of their work off of his foundation, see e.g. this 1982 article by Keohane which laid the groundwork for After Hegemony, and which cites Buchanan alongside Coase.
Here is his Wikipedia page. Here is his Google Scholar page. Here is his 1986 Nobel Memorial Prize lecture, which concludes with the question which motivated Buchanan's entire career:
How can we live together in peace, prosperity, and harmony, while retaining our liberties as autonomous individuals who can, and must, create our own values?