My wife once bought an 8 GB iPhone 4, and regretted it pretty much immediately. That was a horrible experience, and I was stunned that apple would sell such a thing.
So far I've learned it pans out this way for iOS devices:
- 16 GB: the bare minimum for any sort of user. Don't expect to be taking a lot of pictures or use a lot of apps without a lot of storage twiddling.
- 32 GB: fine for regular app use, avoid if you want to take a lot of pictures. I find this to be the sweet spot for coffee table iPads.
- 64 GB: the baseline for people who take a lot of pictures and videos. Anything less and there's too much storage management involved.