Recently picked up an 64gb iPad2. I have a 16Gb iPhone4 and its way to small, although I think 32gb would be plenty for most users myself included but 16 is just not enough, when you factor in music, podcast, apps, photos etc it adds up quickly, atleast with the 64gb i dont have to be so restrictive as to what i store on it.
But I wouldnt buy the 16gb, and would be very happy with the 32Gb, 64gb is just 'nice' to have I guess.