I had that very question, James, and to a certain extent still do.
The simple answer is that Brummel was a nobody who rose to the top British high society with an Eton education, a commission in the Prince of Wales's regiment and a small fortune accumulated through his father's embezzlements. (His faher was a minor governmental official who was the son of a valet.)
Contemporaries valued him much higher than we do. Lord Byron wrote that the three greatest men of his age were himself, Napoleon and George Brummell.
From the Wall Street Journal review of the book which encouraged me to read it: "By the time he was 21, Brummell was worshiped and aped by every swell in London (not least the Prince of Wales, 16 years Brummell's senior). They copied how he walked, what he wore, what he ate, what he bought -- in short, how he lived. Celebrated in his day for his drolleries and grace, the Beau was as responsible as any single man for the transformation of English manners from the 18th-century's gruff bluster into the polished artifice for which the country would become renowned. And of course he had a huge influence on the way men dress -- an influence that long outlived him. Clothing historian James Laver wrote without exaggeration that Brummell "dictat[ed] the main lines of male fashion for the whole of Europe for the next hundred years.""
Brummell was a plain dresser -- wool coats, black boots and plain white linen. He appears to have changed the complex colorful dress of society by the force of his personality. And, the heart of the book is a fascinating description of a typical day in the life of a Regency dandy.
Kelly argues that Brummel was the first celebrity, but I liked the story of Brummel's life without the contemporary implications. Basically he ruled Regency high life, and the Prince of Wales in particular for 17 years.
A very quick and interesting read -- great beach reading for anyone interested in British history.