I know exactly what you are talking about there. There have been just a few occasions when I have finished writing a piece of prose or poetry when I almost can't remember actually typing it in. It's almost as thought the words flowed from somewhere outside of me through my fingertips onto the keyboard.
I'm reminded of something that John McCutcheon said on stage once before performing his song 'Christmas in the Trenches', about the Christmas truce in 1914. He said something along the lines of "There are some songs that you write, and there are some that you just write down. They sort of come through you. And that was definitely the case with this song."
It certainly gives me a great deal of satisfaction when that happens when I write something. It also gives me a huge sense of wonder. It makes me realise that I truly am a writer. But even when I have written something that makes me feel like that, I'll usually find that I will go back at a later stage and tweak it, if only ever so slightly. The chances are that John McCutcheon also tweaked his masterpiece after the initial draft.
But usually (most of the time, if I am to be honest) it's not like that, and I do struggle putting thoughts into words. I'll look at what I've written and think 'That's not how I wanted to express this idea.' It's seldom I'll actually delete a piece of work, though, and so I have a folder full of what I call Scribbles. Occasionally I will dig through this folder hoping to find that one bit of dross that really only needs that final cut and polish to turn it into the gemstone that I knew it was capable of becoming when I had tentatively typed in the first few words.
Thanks for a thought provoking blog entry, @Comicality!