Pendleton certainly made his mark, however, as, in addition to the 38 Executioner novels he wrote himself, more than five-hundred additional novels have been written by various writers and published under published by Gold Eagle (a division of Harlequin).
Last summer, out of curiosity, I purchased the first, the original novel. It was a little better than I expected, but not much. The story is fairly ridiculous and Pendleton's prose is terrible, but I can see how it would appeal to a certain market. I read it then put it on the shelf and forgot about it.
Recently, however, in a box of free books someone was throwing out after a garage sale, amidst tomes on wildlife and hunting and fishing, I found the above books: the first five Executioner novels, plus a later book--Mack Bolan's War Book--that contains a rough chronology of the first 60-some-odd novels, plus lists of weapons he's used and character he's met. I rarely turn down free books, so I scooped them up.
I've re-read the first novel and read the second one now and I have to say, my opinion of the books is actually worse than before. They are silly to the point of nearly being parody of tough-guy heroes with more bullets than brains. I know, from his own writings on the matter, though, that Pendleton did not intend them as such. He saw his character as an American version of James Bond - tough and principled and unwilling to bend from his own code of honor. He also, of course, called himself "the father of action adventure fiction", as if pulp heroes like Doc Savage, The Avenger and the Shadow didn't exist before he was even born.
These books are not intellectual and they are barely literate--thoough some later novels, by other writers, are apparently much better, I'm told--but I can see the market they were meant for and obviously they were insanely successful in the long-run. I don't know of any other book series with 500+ entries.
If nothing else, Pendleton left his mark on the world.