I use my iPad heavily. I've got PDF's of pretty much every source book (running 5-20 Mb each) queued up in Goodreader, and this is a HUGE help at the table. I also keep some character info in the Notes app but although it's easy to edit and never gets lost, it's not an ideal way to organize anything. Lately I've started keeping a few DOCX char sheets in Dropbox and that's handy too but not a game-changer like the PDF source books.
I've looked at a bunch of the iOS utility apps for D&D, including initiative trackers and mapping tools but so far haven't seen anything that feels like it solves a real problem for me at the table.
As far as *which* device to get, imho the tablet form factor is PERFECT for the table. Take a look at the iOS apps available for RPG play in iTunes... if those things make you drool, then an iPad is going to be a clear win. Otherwise I think the main differentiators for brand of tablet will be screen size and battery life... and on that score, the iPad wins again.
I'm a big fan of Android, but the Android tablets aren't there yet. Samsung Galaxy 10.1 has the right hardware specs, but TERRIBLE battery life and the charge ratio is insane, takes over 8 hrs to charge from ~20% to 100% and then only 3-4 hrs to go back down to ~20%. The Kindle Fire looks like it could be a great PDF reader for the source books but I haven't held one in my hands so your guess is as good as mine.