I'm not. I honestly can't even imagine what I would do with 128-256GB. What will you do? Store a library of HD movies to watch on a 3.5" screen? My 16GB is just mind. I MIGHT consider a 32GB model for my next one, to have some leeway for photos/videos taken by the camera- but thats the only thing I can think of that would take up substantial space. 256GB? Give me a break.
The storage space is what got me to upgrade my iPhone 4 when my eligibility date arrived. We'll probably have the 64GB for at least the next phone and then maybe it will go up again, I hope!