I have 2 IPads with 64 GB. My Ipad one is now a dedicated music Ipad--replaces a dying Touch. I have a lot of my own music on it but also lots of downloads from Rhapsody which I play in the car. It has only music apps on it plus mail, Safari and Kindle. My Ipad 3 is about 3/4 full. I have many apps, have a lot of content plus access to Dropbox. My Goodreader app flutuates as I move things to other apps, delete or move to cloud storage (Dropbox, but Icloud also). I have lots of photos--I upload all my raw photo files from SD cards to it, quick process and save jpeg, delete original (have uploaded to main PC, or if traveling, upload from SD card after travel using Ipad for backup). I would be happy for more storage on future Ipads as I use it for just about everything but photo processing, major xls creation by using combos of multiple apps. With the addition of the Apple BT keyboard I have sidelined my laptop and only use a very powerful PC and the Ipad--so files move off and on the Ipad as I use them, create them or just wish to enjoy them.
Having an Ipad for 2.75 years has totally changed the way I use a computer and I see it continuing to change as the tablets change in the future. More storage, faster processor, wonderful screens---lighter, thinner LOL. Its a very personal device and so storage size can vary quite a lot but I think for now for most people the 32 GB is the sweet spot for size unless, like me, you have lots of files moving around, lots of large photo files.