I bought the 16 gig and it's too small. I will buy 32 or 64 next time. I only have about 1 gig of music but I have an 11 mos old I am constantly taking pics and videos of. I have to offload them at least every 3rd day. Plus I'm an app whore. I have like 5 gigs worth. Not to mention the iOS takes up like 3.5 gig. So yeah definitely bigger next time.
I went with 32 gb this time around. With the iPhone 4 I had the 16 and it was always full. I have the 16 gb iPad too which was a mistake, I just couldn't bring myself to spend more than $500 on it. Maybe I will when the iPad 3 comes out.