Tricky question. I find that it shifts...
6 months ago I was doing a lot of traveling on business - so I used it to keep up with my e-mails, call home, and listen to music and audio books. Oh - and the map function was quite handy.
Two months ago I was unemployed - so it got used as a phone lots (10x as much talk time than normal according to the usage figures). And the maps were again handy finding interview locations (and even showing taxi drivers where they were supposed to go).
Now in a new (and better paid) job I commute by train - so that web browser gets a big workout as I check news and other sites.
Point is the iPhone did it all. No problem. And now the Apps open up even greater possibilities. I expect usage will continue to shift and change as time goes on. Importantly I've got enough usage out of it to justify (to me) the cost of the device (£269) and the O2 contract (£35 a month).