I bought my iPad with 32Gb based on my experience with a 16Gb iPod Touch that filled up in no time. I thought that 32Gb would keep me happy for a while.
However, within a couple of months I found myself consistently running out of space and wondered if I should have gone with the 64Gb iPad instead. (Why didn't I? Because I bought the 3G model at $729US. Yes, 64Gb would only have been $100 more, but $829US exceeded the mental threshold of what I was willing to spend on the gadget)
Anyway, I came to the realization that even if I had owned the 64Gb model, I'd probably still be moaning about the lack of space, just a few months later. Instead I decided to look at what was taking up the most room on the device and see if I couldn't deal with it from that angle.
It turns out that movies were my single largest space-wasters. I also realized that I almost always watched them at home, so why was I carrying them around with me on the device? Instead I set up a small home-media server, stashed all my movies there and watch them from the iPad with a compatible streaming app. The result is that the iPad has much more free space and the rest of the family can also watch the movies with other devices. (In my case, the movies were encoded DVDs. If the majority of your movies were purchased from iTunes, etc. you'll need to factor DRM issues into the streaming solution you choose. Maybe look at AirPlay's new features or AirVideo, etc.)
That being said, there are still a few large video files I do carry around on the device, but they tend to be things like tech-conference videos and screencasts -- as such, they usually get deleted shortly after I watch them.
So for me it turns out that 32Gb is still the sweet spot. If I get another iPad in the near future, I'll probably stick with the same capacity.
Sorry for the way-too-long post.