Edudemic has an article defending the use of mobile devices to write full, coherent essays.
My first or second generation Kindle (with the small domed-keys below the screen) is not great for long writing, but that is partially because it is hard to access numbers or special characters, including the comma and question mark. I have not used Google Docs with it (I am not sure you can as Docs requires multiple windows open) but another problem is transferring the completed work. There is no Wordpad or the like that is easily copy-and-pastable into other devices. Still, the Kindle is great for holding nearly complete documents and proofreading and annotating them.
The Kindle was never intended for content creation. How about the iPad? The Edudemic article discusses one concern I had (I have touched an iPad, but little more); keeping your fingers floating above the keyboard. Apparently practice makes that problem less intrusive.