I feel the same way in some way every time something comes along like this. There will always be something better down the line. The question is, what do you stand to gain NOW if you get this NOW?
It's simpler if you don't have it yet.
In my case, I currently have the 16-35 ver 1. I LOVE IT. I have made thousands of great images with it. I got a great deal on it when I bought it. So should I replace it with a version II? I probably won't but I will say that since I got the 24-70 ver II, having my cherished 16-35 with the same size filter would be a nice. Having a wide lens like this is great for telling the story in the image, getting lots of people in the shot and showing a big picture of what's going on. It's NOT necc for landscapes. I'm not a landscape photographer but I know enough to know that a ultra wide lens isn't a good landscape lens due to the distortion that happens at the edges.
I would say that if you have never used a truly wide lens, the 16-35 is a great lens to learn with. It is extremely versatile as long as you are willing to get close, really close to your subjects. It's wonderful indoors and in tight spaces. But if you are shy, stick with the 24 and up lenses, zoom or otherwise.