Isaac Asimov and the Radio Shack TRS-80 Color Computer (1982):
Though you can never be sure whether celebrities actually use the things they endorse, that wasn't an issue with Isaac Asimov's ads for the TRS-80. The science fiction master bought one of Radio Shack's classic PCs in 1981 and was delighted with it, according to "Isaac Asimov: A Life of the Grand Master of Science Fiction" by Michael White.
"He had always complained that electric typewriters moved too slowly for his brain. With this new computer he decided that it was just fast enough to keep up with him", White writes.