e-mail, web browsing, books, I have a few hundred work related files, though most I just open via iDisk as needed.
If you and afford it I'd get at least 32 GB (I did and am wishing I got the 64 GB as I already have 16GB on board (though admittedly I could delete most of the music).
I do find 3G useful, because it is the mobility of the device that is so great. (I guess though that depends a little about where you live and the availability of WiFi hotspots.
If I had to do my purchase all over again, I would have got the 3G version. I ended up getting the Sprint Overdrive though, since I am close to a 4G tower some how.