I was ten when the movie came out, my dad encouraged me to watch it years later when it played on television. I was hooked...
Brando is uproarious as the American GI in Europe, an egotist, sometimes crude, a hustler, preying on gorgeous woman through sympathy, his good looks and his almost pathetic hilarious charm....David Niven is the slick, cool, rich charming Englishman and yes, he's also as conniving as Brando. Though Niven is a bit smarter than "Freddy" (Brando) - It makes for a weird but funny and brief partnership between Niven and Brando, to seduce & fleece some very rich, sometimes beautiful, but always naive, trusting & very willing women.
And thus begins an even funnier competition between Brando and Niven, as the movie really gets rolling along...as each character underestimates the other in cons and setups, then untimely outwits the other...all the way until the final twist of an ironic and funny ending to the film.
Yes, there are indeed a couple of silly and corny scenes...but overall the movie is a very funny farce, as enjoyable & well written comedy to come out of that early 60's era, (1964) or any era for that matter....Brando is truly terrific - Accept no substitutes, i.e., "Dirty Rotten Scoundrels"!