I'm left-handed and the basic Platinum Preppy and the blue ink that comes with it by default work great for me. My hand brushes over the text but the ink dries so fast it's not a problem. Some fancier pens with flexible nib that gives more control over line width really need that you don't push against the grain and are impossible to write with left hand.
There are many left-handed fountain pen users. For extensive discussion of pens, inks, and writing techniques, search for left-handed|lefty at site:www.fountainpennetwork.com.
It mostly seems like they suck for us lefties and the articles about how to use them are just attempts to sell more fountain pens to lefties. The main point of superiority for fountain pens is if you are the person selling fountain pens.
A lot of ballpoints absolutely suck for lefties too, but you can get good ballpoints that work perfect for left handed writing for 30x-100x less than the fountain pens in the article. And not have to jump through hoops or go through any hassle to get them to work.
I would love to hear from some fellow lefties. I love pens, ink, and paper, but I feel cursed.