I use an iPad2, which I use in exactly the same way as I used my Palm Tungsten E2 and my iPod Touch:
1) Writing down ideas when I'm away from my books.
2) Fleshing out ideas when I have all of my books
3) At the table, as DM, it contains the campaign and encounter data
4) at the table as a player, it serves as my character sheet.
5) occasionally, the iPod Touch or iPad2 will be used to look something up on the Internet (like something here, at WotC, the SRD, or Wikipedia) or play a bit of appropriate music.
As of yet, I have not downloaded a single app- all of this is done with the apps that come preinstalled on the machines.