I play mainly Kayle and Teleport/Flash used to be standard on her for me. One day I tried Teleport/Ghost. Haven't gone back to Flash since. In my opinion, on champions like Kayle and Nasus who apply damage in a sustained manner as opposed to burst, Ghost is far superior. When it comes to securing kills, Ghost is the way to go hands down, it has a very low CD and it counters Flash meaning that even if your opponent Flashes your all-in, you're highly likely to catch him anyway. The only thing Flash does better than Ghost is provide escape since you can get over walls and juke opponents with it. But even so, Ghost is not slouch in providing an escape if you're smart about it.
To be honest, I'm surprised they haven't nerfed it yet. It's definitely one of the most powerful spells in the game. I hope they take forever like they are taking to nerf Graves and Fiora. I love Ghost.