This comment by JohnDoey (reproduced in full without permission) on the Asymco article The new feeds and speeds: iPad vs. MacBook Air and iMac sums up the iPad writing experience beautifully (although I think the iPad is a good production system too):

I’m a writer also. You missed a key point about writing on iPad.

Writers who are using iPads are not using them like Macs (or Mac clones,) they are using them like typewriters. A writer with an iPad is exactly the same as a writer with a typewriter or a filmmaker with a movie camera or a singer with a microphone. Those are capture devices, not editing devices. You just write, shoot, or sing with as few interruptions and as little technical overhead as possible.

A Mac is a production system, an editing system. A mouse is an editing device. That is the opposite of a capture system like iPad or a typewriter or a camera or a microphone.

In the past, writers had to either write directly into the Mac production system or use a typewriter and become good at OCR scanning because there were no digital typewriters. Now there are.

So you have the option today of either continuing to write with digital production tools on a Mac or you can write with digital writing tools on iPad, or you can do a mix of both. What you can’t do is turn an iPad into a production system. That is the last thing it wants to be. It deliberately left the OS X production tools behind on the Mac in the same way that iPod left its music management tools back on the Mac. If you need a mouse, you use it on your Mac. We already had that. If you need a typewriter, now you can get one on an iPad instead of having to buy the whole dedicated device, and the iPad version is digital same as iPad cameras and musical instruments are digital, so it is easy to move work from your iPad typewriter to your Mac for production and editing.