Of the Mount Vesuvius:
I am glad I visited it, partly because it was well worth it, and chiefly because I shall never have to do it again.
Sometimes misquoted as: I'm glad I did it, partly because it was worth it, but mostly because I shall never have to do it again.
Mr. Clemens and Mark Twain: A Biography (Simon & Schuster, Dec 15, 1991), p. 48