All of the above!
Sadly I only have the Wifi versions at the moment but want to get a 3G shortly.
Its keyboard is not as good as a full size one but for data input absolutely fine.
Readability of the screen is fine and can get enhanced. The reliability and battery life is second to none.
They are the way forward and certainly if you fly in the States and therefore can use apps like Foreflight to their full potential indicate that anything else is just a poor second best (at best!)