I actually tried to game like this once, just as an experiment. Most games, of course, don't do well. For FPSs (the ones that actually work with 9:16, that is), it's an interesting experience. The resolution appears much higher, as it's basically only rendering what would normally be the middle ~third of the screen, but that's also the obvious downside: your FOV is now only like <30°. Which, for an FPS......yeah. Less than ideal.
Portrait gaming is good for pinball games and some shoot-'em-ups designed for 9:16. I'd certainly like to hear of more exceptions, though.